International Statistical Classification of Diseases and Related Health Problems, mostly known as "ICD", assigns codes to classify diseases, symptoms, complaints, social behaviors, injuries, and such medical-related findings.

ICD 10 classifies adjustment disorders under F40-F48 and under neurotic, stress-related and somatoform disorders.

An adjustment disorder (AD)—sometimes called exogenous, reactive, or situational depression—occurs when an individual is unable to adjust to or cope with a particular stress or a major life event. Since people with this disorder normally have symptoms that depressed people do, such as general loss of interest, feelings of hopelessness and crying, this disorder is sometimes known as situational depression. Unlike major depression, the disorder is caused by an outside stressor and generally resolves once the individual is able to adapt to the situation. One hypothesis about AD is that it may represent a sub-threshold clinical syndrome.

The condition is different from anxiety disorder, which lacks the presence of a stressor, or post-traumatic stress disorder and acute stress disorder, which usually are associated with a more intense stressor.

Common characteristics of AD include mild depressive symptoms, anxiety symptoms, and traumatic stress symptoms or a combination of the three. There are nine types of AD listed in the DSM-III-R. According to the DSM-IV-TR, there are six types of AD, which are characterized by the following predominant symptoms: depressed mood, anxiety, mixed depression and anxiety, disturbance of conduct, mixed disturbance of emotions and conduct, and unspecified. However, the criteria for these symptoms are not specified in greater detail. AD may be acute or chronic, depending on whether it lasts more or less than six months. According to the DSM-IV-TR, if the AD lasts less than 6 months, then it may be considered acute. If it lasts more than six months, it may be considered chronic. Moreover, the symptoms cannot last longer than six months after the stressor(s), or its consequences, have terminated. Diagnosis of AD is quite common; there is an estimated incidence of 5%–21% among psychiatric consultation services for adults. Adult women are diagnosed twice as often as are adult men. Among children and adolescents, girls and boys are equally likely to receive this diagnosis. AD was introduced into the psychiatric classification systems almost 30 years ago, but similar syndromes were recognized for many years before that.

Depressive Disorder Not Otherwise Specified (DD-NOS) is designated by the code "311" in the DSM-IV for depressive disorders that are impairing but do not fit any of the officially specified diagnoses. According to the DSM-IV, DD-NOS encompasses "any depressive disorder that does not meet the criteria for a specific disorder." In the DSM-5, it is called unspecified depressive disorder.

Examples of disorders in this category include those sometimes described as minor depressive disorder and recurrent brief depression.

"Depression" refers to a spectrum of disturbances in mood that vary from mild to severe and from short periods to constant illness. DD-NOS is diagnosed if a patients symptoms fail to meet the criteria more common depressive disorders such as major depressive disorder or dysthymia. Although DD-NOS shares similar symptoms to dysthymia, dysthymia is classified by a period of at least 2 years of constantly recurring depressed mood, where as DD-NOS is classified by much shorter periods of depressed moods.

For most people who suffer the condition, their life will be significantly affected. DD-NOS can make many aspects of a person's daily life difficult to manage, inhibiting their ability to enjoy the things that used to make them happy. Sufferers of the disorder tend to isolate themselves from their friends and families, lose interest in some activities, and experience behavioural changes and sleeping disorders. Some sufferers also experience suicidal tendencies or suicide attempts. In addition to having these symptoms, a diagnosis of DD-NOS will only be made if the symptoms cause significant distress or impairment in social, occupational, or other important areas of functioning. For the diagnosis to be accurate, a psychiatrist is required to spend extensive time with the patient.

Symptoms of the disorder may arise due to several reasons. These include:

- Distress due to medical conditions

- Environmental effects and situations

However, the effects of drugs or medication or bereavement are not classified under the diagnosis.

A person will not be diagnosed with the condition if they have or have had any of the following: a major depressive episode, manic episode, mixed episode or hypomanic episode.

A diagnosis of the disorder will look like: "Depressive Disorder NOS 311".

According to Shear et al. (2011):

- The person has been bereaved (i.e. experienced the death of a loved one) for at least six months

- At least one of the following symptoms of persistent, intense, acute grief has been present for a period longer than is expected by others in the person’s social (or cultural) environment:

- Persistent intense yearning or longing for the person who died

- Frequent intense feelings of loneliness, or that life is empty or meaningless without the person who died

- Recurrent thoughts that it is unfair, meaningless or unbearable to live when a loved one has died, or a recurrent urge to die in order to find (or join) the deceased

- Frequent preoccupying thoughts about the person who died; e.g. thoughts or images of the person intrude on activities or interfere with functioning

- At least two of the following symptoms are present for at least one month:

- Frequent, troubling rumination about the circumstances (or consequences) of the death (concerns about how or why the person died, about not being able to manage without their loved one, thoughts of having let the deceased person down, etc.)

- Recurrent feeling of disbelief or inability to accept the death

- Persistent feeling of shock; feeling stunned, dazed or emotionally numb since the death

- Recurrent feelings of anger or bitterness related to the death

- Persistent difficulty trusting or caring about other people, or envy of others who have not experienced a similar loss

- Frequently experiencing pain (or other symptoms) that the deceased person had, hearing the voice of (or seeing) the deceased person

- Experiencing intense emotional or physiological reactivity to memories of the person who died or to reminders of the loss

- Changes in behavior due to avoidance (or its opposite, excessive proximity-seeking—refraining from going places, doing things, or having contact with things that are reminders of the loss; feeling drawn to reminders of the person—wanting to see, touch, hear or smell things to feel close to the person who died). Both symptoms may coexist in the same individual.

- Duration of symptoms and impairment of at least one month

- Symptoms cause clinically significant distress or impairment in social, occupational or other major areas of functioning, where impairment is not explicable as a culturally appropriate response

311- Depressive Disorder Not Otherwise Specified (NOS)

The Depressive disorder NOS category includes disorders with depressive features that do not meet the criteria for Major Depressive Disorder, Dysthymic disorder, Adjustment Disorder with Depressed Mood or Adjustment Disorder with Mixed Anxiety and Depressed Mood. Sometimes depressive symptoms can present as part of an Anxiety Disorder Not otherwise Specified. Examples of Depressive Disorder Not Otherwise Specified include.

- Premenstrual Dysphoric Disorder: in most menstrual cycles during the past years, (e.g., markedly depressed mood, marked anxiety, marked affective lability, or decreased interest in activities) regularly occurred during the onset of menses. These symptoms must be severe enough to markedly interfere with work, school, or usual activities and be entirely absent for at least 1 week post menses.

- Minor depressive disorder: episodes of at least 2 weeks of depressive symptoms but with fewer than the five items required for Major Depressive Disorder.

- Recurrent brief depressive disorder: depressive episodes lasting from 2 days up to 2 weeks, occurring at least once a month for 12 months (not associated with the menstrual cycle)

- Post psychotic depressive Disorder of schizophrenia: a Major Depressive Episode that occurs during the residual phase of schizophrenia.

- A Major Depressive Episode superimposed on delusional disorder, Psychotic Disorder Not Otherwise Specified, or the active phase of schizophrenia.

- Situations in which the clinician has concluded that a depressive disorder is present but is unable to determine whether it is primary, due to a general medical condition, or substance induced.

Symptoms of the disorder may arise due to several reasons. These include:

- Distress due to medical conditions

- Environmental effects and situations

Complicated grief is considered when an individual’s ability to resume normal activities and responsibilities is continually disrupted beyond six months of bereavement. Six months is considered to be the appropriate point of CGD consideration, since studies show that most people are able to integrate bereavement into their lives by this time.

Symptoms of PTSD generally begin within the first 3 months after the inciting traumatic event, but may not begin until years later. In the typical case, the individual with PTSD persistently avoids trauma-related thoughts and emotions, and discussion of the traumatic event, and may even have amnesia of the event. However, the event is commonly relived by the individual through intrusive, recurrent recollections, flashbacks, and nightmares. While it is common to have symptoms after any traumatic event, these must persist to a sufficient degree (i.e., causing dysfunction in life or clinical levels of distress) for longer than one month after the trauma to be classified as PTSD (clinically significant dysfunction or distress for less than one month after the trauma may be acute stress disorder).

Posttraumatic embitterment disorder (PTED) is a proposed disorder modeled after posttraumatic stress disorder. Some psychiatrists are proposing this as a mental disorder because they believe that there are people who have become so bitter, they can barely function. PTED patients do not fit the formal criteria for PTSD and can be clinically distinguished from it, prompting the description of a new and separate disorder.

German psychiatrist Michael Linden, who has conducted research on the proposed disorder, describes its effect on people: "They feel the world has treated them unfairly. It's one step more complex than anger. They're angry plus helpless." He says that people with the disorder are almost treatment resistant and that; "These people usually don't come to treatment because 'the world has to change, not me. He believes that 1 to 2 percent of people are affected at any given time, and explains that, although sufferers of the disorder tend to have a desire for vengeance, "...Revenge is not a treatment."

PTED so far has no official status and is not listed in the DSM-IV-TR or ICD-10-CM.

Cyclothymia is characterized by short cycles of baseline, stable periods of not over two months and numerous swings between depression and hypomania that fail to meet the severity of sustained duration criterion for major affective syndromes for at least two years.

Depressive/dysthymic episodes. Symptoms of the "depressive/dysthymic phase" may include any of the following conditions:difficulty making decisions, problems concentrating, poor memory recall, guilt, self-criticism, low self-esteem, pessimism, self-destructive thinking, constant sadness, apathy, hopelessness, helplessness and irritability. Also common are quick temper, poor judgment, lack of motivation, social withdrawal, appetite change, lack of sexual desire, self-neglect, fatigue, insomnia and sleepiness.

Hypomanic episodes. Symptoms of the "hypomanic episode" may include any of the following conditions: unusually good mood or cheerfulness (euphoria), extreme optimism, inflated self-esteem, rapid speech, racing thoughts, aggressive or hostile behavior, lack of consideration for others, agitation, massively increased physical activity, risky behavior, spending sprees, increased drive to perform or achieve goals, increased sexual drive, decreased need for sleep, tendency to be easily distracted, and inability to concentrate.

Physical symptoms include:

- palpitations

- anorexia

- dry mouth

- insomnia

- thoracic/chest pressure

- respiratory difficulties

- epigastric mass

- headache

- a whole-body sensation of heat (distinct from heat intolerance, a symptom of hyperthyroidism)

Psychological symptoms include:

- being easily startled

- externalization of anger, also known in Korean as "bun" (분, 憤, "eruption of anger"), a Korean culture-related sentiment related to social unfairness

- generally sad mood

- frequent sighing

- a feeling of "eok-ul" (억울, 抑鬱, [feeling of] unfairness)

- being easily agitated

- feelings of guilt

- feelings of impending doom

Diagnosed patients may also have a medical history of prior major depressive disorder, dysthymic disorder, anxiety disorders, somatoform disorders, or adjustment disorder according to the "Diagnostic and Statistical Manual of Mental Disorders", fourth edition (DSM-IV) criteria.

Diagnosed patients are most likely to be middle-aged, post-menopausal women with low socio-economic status.

The following are the revised criteria for a diagnosis of cyclothymic disorder (DSM-IV-TR 301.13) from the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV-TR):

- A. For at least 2 years (1 year in children and adolescents), the presence of numerous periods with hypomanic symptoms and numerous periods with depressive symptoms that do not meet criteria for a Major Depressive Episode

- B. During the above period, the person has not been without the symptoms in A for more than 2 months in the 2-year period

- C. No Major Depressive Episode, Manic Episode, or Mixed Episode has been present during the first 2 years of the disturbance.

- D. The symptoms in Criterion A are not better accounted for by Schizoaffective Disorder and are not superimposed on Schizophrenia, Schizophreniform Disorder, Delusional Disorder, or Psychotic Disorder Not Otherwise Specified.

- E. The symptoms are not due to the direct physiological effects of a substance (e.g. drug of abuse, a medication) or a general medical condition (e.g., hyperthyroidism).

- F. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.

The DSM-IV-TR notes that the mood disturbance must be not severe enough to constitute a full-fledged manic or major depressive episode. The diagnosis requires that there be periods of both hypomania and depression and that periods of normal mood not last longer than 2 months.

The DSM-IV-TR also notes that Cyclothymia and borderline personality disorder share similar features and that individuals can be diagnosed with both BPD and Cyclothymia depending on the symptoms they present.

Posttraumatic stress disorder (PTSD) is a mental disorder that can develop after a person is exposed to a traumatic event, such as sexual assault, warfare, traffic collisions, or other threats on a person's life. Symptoms may include disturbing thoughts, feelings, or dreams related to the events, mental or physical distress to trauma-related cues, attempts to avoid trauma-related cues, alterations in how a person thinks and feels, and an increase in the fight-or-flight response. These symptoms last for more than a month after the event. Young children are less likely to show distress but instead may express their memories through play. A person with PTSD is at a higher risk for suicide and intentional self-harm.

Most people who have experienced a traumatic event will not develop PTSD. People who experience interpersonal trauma (for example rape or child abuse) are more likely to develop PTSD, as compared to people who experience non-assault based trauma such as accidents and natural disasters. About half of people develop PTSD following rape. Children are less likely than adults to develop PTSD after trauma, especially if they are under ten years of age. Diagnosis is based on the presence of specific symptoms following a traumatic event.

Prevention may be possible when therapy is targeted at those with early symptoms but is not effective when carried out among all people following trauma. The main treatments for people with PTSD are counselling and medication. A number of different types of therapy may be useful. This may occur one-on-one or in a group. Antidepressants of the selective serotonin reuptake inhibitor type are the first-line medications for PTSD and result in benefit in about half of people. These benefits are less than those seen with therapy. It is unclear if using medications and therapy together has greater benefit. Other medications do not have enough evidence to support their use and in the case of benzodiazepines may worsen outcomes.

In the United States about 3.5% of adults have PTSD in a given year, and 9% of people develop it at some point in their life. In much of the rest of the world, rates during a given year are between 0.5% and 1%. Higher rates may occur in regions of armed conflict. It is more common in women than men. Symptoms of trauma-related mental disorders have been documented since at least the time of the ancient Greeks. During the World Wars study increased and it was known under various terms including "shell shock" and "combat neurosis". The term "posttraumatic stress disorder" came into use in the 1970s in large part due to the diagnoses of U.S. military veterans of the Vietnam War. It was officially recognized by the American Psychiatric Association in 1980 in the third edition of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-III).

A mental breakdown (also known as a nervous breakdown) is an acute, time-limited mental disorder that manifests primarily as severe stress-induced depression, anxiety, or dissociation in a previously functional individual, to the extent that they are no longer able to function on a day-to-day basis until the disorder is resolved. A nervous breakdown is defined by its temporary nature, and often closely tied to psychological burnout, severe overwork, sleep deprivation, and similar stressors, which may combine to temporarily overwhelm an individual with otherwise sound mental functions.

The terms "nervous breakdown" and "mental breakdown" have not been formally defined through a medical diagnostic system such as the DSM-5 or ICD-10, and are nearly absent from current scientific literature regarding mental illness. Although "nervous breakdown" is not rigorously defined, surveys of laypersons suggest that the term refers to a specific acute time-limited reactive disorder, involving symptoms such as anxiety or depression, usually precipitated by external stressors. Many health experts today refer to a nervous breakdown as a "modern mental health crisis."

Specific cases are sometimes described as a "breakdown" only after the emotional and physical demands on a person's life are so great as to prevent him or her from performing activities of daily living or, less strictly, only when those demands prevent him/her from performing his/her familial or occupational duties.

Nervous breakdowns are often caused by serious ongoing mental health disorders.

PTSD has significant arousal and other effects on physiological, endocrine, HPA axis, brain centers, and neurological systems. These have been the bases of treatments well known and studied for decades via both theoretical and practical descriptions of illness, rationales of treatment modalities, and mechanisms of therapeutic actions.

PTED may arouse or influence PTSD-affected systems differently or arouse different systems. Thus PTED, although modeled on PTSD, may differ to various degrees and in various ways.

There is no published work at this time on different underlying neurological, endocrinology, and physiological changes in PTED patients as an analog to PTSD.

The following conditions commonly coexist (comorbid) with dependent personality disorder:

- mood disorders

- anxiety disorders

- adjustment disorder

- borderline personality disorder

- avoidant personality disorder

- histrionic personality disorder

Hwabyeong or Hwabyung is a Korean somatization disorder, a mental illness which arises when people are unable to confront their anger as a result of conditions which they perceive to be unfair.

Hwabyung is a colloquial and somewhat inaccurate name, as it refers to the etiology of the disorder rather than its symptoms or apparent characteristics. Hwabyung is known as a culture-bound syndrome. The word hwabyung is composed of "hwa" (the Sino-Korean word for "fire" which can also contextually mean "anger") and "byung" (the Sino-Korean word for "syndrome" or "illness"). In South Korea, it may also be called "ulhwabyeong" (), literally "depression anger illness". In one survey, 4.1% of the general population in a rural area in Korea were reported as having hwabyung. Another survey shows that about 35% of Korean workers are affected by this condition at some time.

One of the symptoms of conduct disorder is a lower level of fear. Research performed on the impact of toddlers exposed to fear and distress shows that negative emotionality (fear) predicts toddlers' empathy-related response to distress. The findings support that if a caregiver is able to respond to infant cues, the toddler has a better ability to respond to fear and distress. If a child does not learn how to handle fear or distress the child will be more likely to lash out at other children. If the caregiver is able to provide therapeutic intervention teaching children at risk better empathy skills, the child will have a lower incident level of conduct disorder.

Major depression significantly affects a person's family and personal relationships, work or school life, sleeping and eating habits, and general health. Its impact on functioning and well-being has been compared to that of other chronic medical conditions such as diabetes.

A person having a major depressive episode usually exhibits a very low mood, which pervades all aspects of life, and an inability to experience pleasure in activities that were formerly enjoyed. Depressed people may be preoccupied with, or ruminate over, thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness, hopelessness, and self-hatred. In severe cases, depressed people may have symptoms of psychosis. These symptoms include delusions or, less commonly, hallucinations, usually unpleasant. Other symptoms of depression include poor concentration and memory (especially in those with melancholic or psychotic features), withdrawal from social situations and activities, reduced sex drive, irritability, and thoughts of death or suicide. Insomnia is common among the depressed. In the typical pattern, a person wakes very early and cannot get back to sleep. Hypersomnia, or oversleeping, can also happen. Some antidepressants may also cause insomnia due to their stimulating effect.

A depressed person may report multiple physical symptoms such as fatigue, headaches, or digestive problems; physical complaints are the most common presenting problem in developing countries, according to the World Health Organization's criteria for depression. Appetite often decreases, with resulting weight loss, although increased appetite and weight gain occasionally occur. Family and friends may notice that the person's behavior is either agitated or lethargic. Older depressed people may have cognitive symptoms of recent onset, such as forgetfulness, and a more noticeable slowing of movements. Depression often coexists with physical disorders common among the elderly, such as stroke, other cardiovascular diseases, Parkinson's disease, and chronic obstructive pulmonary disease.

Depressed children may often display an irritable mood rather than a depressed mood, and show varying symptoms depending on age and situation. Most lose interest in school and show a decline in academic performance. They may be described as clingy, demanding, dependent, or insecure. Diagnosis may be delayed or missed when symptoms are interpreted as normal moodiness.

Dependent personality disorder (DPD), formerly known as asthenic personality disorder, is a personality disorder that is characterized by a pervasive psychological dependence on other people. This personality disorder is a long-term condition in which people depend on others to meet their emotional and physical needs, with only a minority achieving normal levels of independence.

The cause of dependent personality disorder is unknown. A study in 2012 estimated that between 55% and 72% of the risk of the condition is inherited from one's parents. The difference between a "dependent personality" and a "dependent personality disorder" is somewhat subjective, which makes diagnosis sensitive to cultural influences such as gender role expectations.

Depression is a state of a low mood and aversion to activity that can affect a person's thoughts, behavior, feelings, and sense of well-being. A depressed mood is a normal temporary reaction to life events such as loss of a loved one. It is also a symptom of some physical diseases and a side effect of some drugs and medical treatments. Depressed mood is also a symptom of some mood disorders such as major depressive disorder or dysthymia.

People with a depressed mood may be notably sad, anxious, or empty; they may also feel notably hopeless, helpless, dejected, or worthless. Other symptoms expressed may include senses of guilt, irritability, or anger. Further feelings expressed by these individuals may include feeling ashamed or an expressed restlessness. These individuals may notably lose interest in activities that they once considered pleasurable to family and friends or otherwise experience either a loss of appetite or overeating. Experiencing problems concentrating, remembering general facts or details, otherwise making decisions or experiencing relationship difficulties may also be notable factors in these individuals' depression and may also lead to their attempting or actually dying by suicide.

Expressed insomnia, excessive sleeping, fatigue, and vocalizing general aches, pains, and digestive problems and a reduced energy may also be present in individuals experiencing depression.

Children with conduct disorder have a high risk of developing other adjustment problems. Specifically, risk factors associated with conduct disorder and the effects of conduct disorder symptomatology on a child's psychosocial context have been linked to overlap with other psychological disorders. In this way, there seems to be reciprocal effects of comorbidity with certain disorders, leading to increased overall risk for these youth.

Major depression frequently co-occurs with other psychiatric problems. The 1990–92 "National Comorbidity Survey" (US) reports that half of those with major depression also have lifetime anxiety and its associated disorders such as generalized anxiety disorder. Anxiety symptoms can have a major impact on the course of a depressive illness, with delayed recovery, increased risk of relapse, greater disability and increased suicide attempts. There are increased rates of alcohol and drug abuse and particularly dependence, and around a third of individuals diagnosed with ADHD develop comorbid depression. Post-traumatic stress disorder and depression often co-occur. Depression may also coexist with attention deficit hyperactivity disorder (ADHD), complicating the diagnosis and treatment of both. Depression is also frequently comorbid with alcohol abuse and personality disorders.

Depression and pain often co-occur. One or more pain symptoms are present in 65% of depressed patients, and anywhere from 5 to 85% of patients with pain will be suffering from depression, depending on the setting; there is a lower prevalence in general practice, and higher in specialty clinics. The diagnosis of depression is often delayed or missed, and the outcome can worsen if the depression is noticed but completely misunderstood.

Depression is also associated with a 1.5- to 2-fold increased risk of cardiovascular disease, independent of other known risk factors, and is itself linked directly or indirectly to risk factors such as smoking and obesity. People with major depression are less likely to follow medical recommendations for treating and preventing cardiovascular disorders, which further increases their risk of medical complications. In addition, cardiologists may not recognize underlying depression that complicates a cardiovascular problem under their care.

Individuals with schizophrenia may experience hallucinations (most reported are hearing voices), delusions (often bizarre or persecutory in nature), and disorganized thinking and speech. The last may range from loss of train of thought, to sentences only loosely connected in meaning, to speech that is not understandable known as word salad. Social withdrawal, sloppiness of dress and hygiene, and loss of motivation and judgment are all common in schizophrenia.

Distortions of self-experience such as feeling as if one's thoughts or feelings are not really one's own to believing thoughts are being inserted into one's mind, sometimes termed passivity phenomena, are also common. There is often an observable pattern of emotional difficulty, for example lack of responsiveness. Impairment in social cognition is associated with schizophrenia, as are symptoms of paranoia. Social isolation commonly occurs. Difficulties in working and long-term memory, attention, executive functioning, and speed of processing also commonly occur. In one uncommon subtype, the person may be largely mute, remain motionless in bizarre postures, or exhibit purposeless agitation, all signs of catatonia. People with schizophrenia often find facial emotion perception to be difficult. It is unclear if the phenomenon called "thought blocking", where a talking person suddenly becomes silent for a few seconds to minutes, occurs in schizophrenia.

About 30 to 50 percent of people with schizophrenia fail to accept that they have an illness or comply with their recommended treatment. Treatment may have some effect on insight.

People with schizophrenia may have a high rate of irritable bowel syndrome but they often do not mention it unless specifically asked. Psychogenic polydipsia, or excessive fluid intake in the absence of physiological reasons to drink, is relatively common in people with schizophrenia.

Premorbidity refers to the state of functionality prior to the onset of a disease or illness. It is most often used in relation to psychological function (e.g. premorbid personality or premorbid intelligence), but can also be used in relation to other medical conditions (e.g. premorbid lung function or premorbid heart rate).