Diagnosis of a specific type of delusional disorder can sometimes be made based on the content of the delusions. The "Diagnostic and Statistical Manual of Mental Disorders" (DSM) enumerates seven types:

- Erotomanic type (erotomania): delusion that another person, often a prominent figure, is in love with the individual. The individual may breach the law as he/she tries to obsessively make contact with the desired person.

- Grandiose type: delusion of inflated worth, power, knowledge, identity or believes themself to be a famous person, claiming the actual person is an impostor or an impersonator.

- Jealous type: delusion that the individual's sexual partner is unfaithful when it is untrue. The patient may follow the partner, check text messages, emails, phone calls etc. in an attempt to find "evidence" of the infidelity.

- Persecutory type: This delusion is a common subtype. It includes the belief that the person (or someone to whom the person is close) is being malevolently treated in some way. The patient may believe that he/she has been drugged, spied upon, harassed and so on and may seek "justice" by making police reports, taking court action or even acting violently.

- Somatic type: delusions that the person has some physical defect or general medical condition

- Mixed type: delusions with characteristics of more than one of the above types but with no one theme predominating.

- Unspecified type: delusions that cannot be clearly determined or characterized in any of the categories in the specific types.

A hallucination is defined as sensory perception in the absence of external stimuli. Hallucinations are different from illusions, or perceptual distortions, which are the misperception of external stimuli. Hallucinations may occur in any of the senses and take on almost any form, which may include simple sensations (such as lights, colors, tastes, and smells) to experiences such as seeing and interacting with fully formed animals and people, hearing voices, and having complex tactile sensations.

Auditory hallucinations, particularly experiences of hearing voices, are the most common and often prominent feature of psychosis. Hallucinated voices may talk about, or to, the person, and may involve several speakers with distinct personalities. Auditory hallucinations tend to be particularly distressing when they are derogatory, commanding or preoccupying. However, the experience of hearing voices need not always be a negative one. One research study has shown that the majority of people who hear voices are not in need of psychiatric help. The Hearing Voices Movement has subsequently been created to support voice hearers, regardless of whether they are considered to have a mental disorder or not.

People with psychosis normally have one or more of the following:

- hallucinations

- delusions

- catatonia

- thought disorder.

Impairments in social cognition also occur.

The following can indicate a delusion:

1. The patient expresses an idea or belief with unusual persistence or force.

2. That idea appears to have an undue influence on the patient's life, and the way of life is often altered to an inexplicable extent.

3. Despite his/her profound conviction, there is often a quality of secretiveness or suspicion when the patient is questioned about it.

4. The individual tends to be humorless and oversensitive, especially about the belief.

5. There is a quality of "centrality": no matter how unlikely it is that these strange things are happening to him/her, the patient accepts them relatively unquestioningly.

6. An attempt to contradict the belief is likely to arouse an inappropriately strong emotional reaction, often with irritability and hostility.

7. The belief is, at the least, unlikely, and out of keeping with the patient's social, cultural, and religious background.

8. The patient is emotionally over-invested in the idea and it overwhelms other elements of their psyche.

9. The delusion, if acted out, often leads to behaviors which are abnormal and/or out of character, although perhaps understandable in the light of the delusional beliefs.

10. Individuals who know the patient observe that the belief and behavior are uncharacteristic and alien.

Additional features of delusional disorder include the following:

1. It is a primary disorder.

2. It is a stable disorder characterized by the presence of delusions to which the patient clings with extraordinary tenacity.

3. The illness is chronic and frequently lifelong.

4. The delusions are logically constructed and internally consistent.

5. The delusions do not interfere with general logical reasoning (although within the delusional system the logic is perverted) and there is usually no general disturbance of behavior. If disturbed behavior does occur, it is directly related to the delusional beliefs.

6. The individual experiences a heightened sense of self-reference. Events which, to others, are nonsignificant are of enormous significance to him or her, and the atmosphere surrounding the delusions is highly charged.

According to the DSM-IV-TR diagnostic criteria for delusional disorders, grandiose-type symptoms include grossly exaggerated beliefs of:

- self-worth

- power

- knowledge

- identity

- exceptional relationship to a divinity or famous person.

For example, a patient who has fictitious beliefs about his or her power or authority may believe himself or herself to be a ruling monarch who deserves to be treated like royalty.

There are substantial differences in the degree of grandiosity linked with grandiose delusions in different patients. Some patients believe they are God, the Queen of England, a president's son, a famous rock star, and so on. Others are not as expansive and think they are skilled sports-persons or great inventors.

In addition to these categories, delusions often manifest according to a consistent theme. Although delusions can have any theme, certain themes are more common. Some of the more common delusion themes are:

- Delusion of control: False belief that another person, group of people, or external force controls one's general thoughts, feelings, impulses, or behavior.

- Cotard delusion: False belief that one does not exist or has died.

- Delusional jealousy: False belief that a spouse or lover is having an affair, with no proof to back up their claim.

- Delusion of guilt or sin (or delusion of self-accusation): Ungrounded feeling of remorse or guilt of delusional intensity.

- Delusion of mind being read: False belief that other people can know one's thoughts.

- Delusion of thought insertion: Belief that another thinks through the mind of the person.

- Delusion of reference: False belief that insignificant remarks, events, or objects in one's environment have personal meaning or significance.

- Erotomania: False belief that another person is in love with them.

- Grandiose religious delusion: Belief that the affected person is a god or chosen to act as a god.

- Somatic delusion: Delusion whose content pertains to bodily functioning, bodily sensations or physical appearance. Usually the false belief is that the body is somehow diseased, abnormal or changed. A specific example of this delusion is delusional parasitosis: Delusion in which one feels infested with insects, bacteria, mites, spiders, lice, fleas, worms, or other organisms. Affected individuals may also report being repeatedly bitten. In some cases, entomologists are asked to investigate cases of mysterious bites. Sometimes physical manifestations may occur including skin lesions.

- Delusion of poverty: Person strongly believes they are financially incapacitated. Although this type of delusion is less common now, it was particularly widespread in the days preceding state support.

Grandiose delusions or delusions of grandeur are principally a subtype of delusional disorder but could possibly feature as a symptom of schizophrenia and manic episodes of bipolar disorder. Grandiose delusions are characterized by fantastical beliefs that one is famous, omnipotent or otherwise very powerful. The delusions are generally fantastic, often with a supernatural, science-fictional, or religious bent. In colloquial usage, one who overestimates one's own abilities, talents, stature or situation is sometimes said to have "delusions of grandeur". This is generally due to excessive pride, rather than any actual delusions. Grandiose delusions or delusions of grandeur can also be associated with megalomania.

The delusions that fall under this category are:

- Capgras delusion: the belief that (usually) a close relative or spouse has been replaced by an identical-looking impostor.

- Fregoli delusion: the belief that various people whom the believer meets are actually the same person in disguise.

- Intermetamorphosis: the belief that people in one's environment swap identities with each other while maintaining the same appearance.

- Subjective doubles: a person believes there is a doppelgänger or double of him- or herself carrying out independent actions.

- Cotard delusion: the belief that oneself is dead or does not exist; sometimes coupled with the belief that one is putrefying or missing internal organs.

- Mirrored-self misidentification: the belief that one's reflection in a mirror is some other person.

- Reduplicative paramnesia: the belief that a familiar person, place, object, or body part has been duplicated. For example, a person may believe that they are, in fact, not in the hospital to which they were admitted, but in an identical-looking hospital in a different part of the country.

- Somatoparaphrenia: the delusion where one denies ownership of a limb or an entire side of one's body (often connected with stroke).

Note that some of these delusions are sometimes grouped under the umbrella term of delusional misidentification syndrome.

Persecutory delusions are the most common type of delusions and involve the theme of being followed, harassed, cheated, poisoned or drugged, conspired against, spied on, attacked, or otherwise obstructed in the pursuit of goals.

Persecutory delusions are a condition in which the affected person wrongly believes that they are being persecuted. Specifically, they have been defined as containing three central elements: The individual thinks that:

1. harm is occurring, or is going to occur.

2. the persecutor(s) has(have) the intention to cause harm.

3. they are constantly being prejudged or profiled.

According to the DSM-IV-TR, persecutory delusions are the most common form of delusions in schizophrenia, where the person believes they are "being tormented, followed, sabotaged, tricked, spied on, or ridiculed." In the DSM-IV-TR, persecutory delusions are the main feature of the persecutory type of delusional disorder. When the focus is to remedy some injustice by legal action, they are sometimes called "querulous paranoia".

Research suggests that the severity of the delusions of grandeur is directly related to a higher self-esteem in individuals and inversely related to any individual’s severity of depression and negative self-evaluations. Lucas "et al." found that there is no significant gender difference in the establishment of grandiose delusion. However, there is a claim that ‘the particular component of Grandiose delusion’ may be variable across both genders. Also, it had been noted that the presence of GDs in people with at least grammar or high school education was greater than lesser educated persons. Similarly, the presence of grandiose delusions in individuals who are the eldest is greater than in individuals who are the youngest of their siblings.

Signs and symptoms of Fregoli's:

- delusions

- visual memory deficit

- deficit in self-monitoring

- deficit in self-awareness

- hallucinations

- deficit in executive functions

- deficit in cognitive flexibility

- history of seizure activity

- epileptogenic activity

When the focus is to remedy some injustice by legal action, they are sometimes called "querulous paranoia".

In cases where reporters of stalking behavior have been judged to be making false reports, a majority of them were judged to be delusional.

A monothematic delusion is a delusional state that concerns only one particular topic. This is contrasted by what is sometimes called "multi-thematic" or "polythematic" delusions where the person has a range of delusions (typically the case of schizophrenia). These disorders can occur within the context of schizophrenia or dementia or they can occur without any other signs of mental illness. When these disorders are found outside the context of mental illness, they are often caused by organic dysfunction as a result of traumatic brain injury, stroke, or neurological illness.

People who experience these delusions as a result of organic dysfunction often do not have any obvious intellectual deficiency nor do they have any other symptoms. Additionally, a few of these people even have some awareness that their beliefs are bizarre, yet they cannot be persuaded that their beliefs are false.

The main symptoms of paraphrenia are paranoid delusions and hallucinations. The delusions often involve the individual being the subject of persecution, although they can also be erotic, hypochondriacal, or grandiose in nature. The majority of hallucinations associated with paraphrenia are auditory, with 75% of patients reporting such an experience; however, visual, tactile, and olfactory hallucinations have also been reported. The paranoia and hallucinations can combine in the form of “threatening or accusatory voices coming from neighbouring houses [and] are frequently reported by the patients as disturbing and undeserved". Patients also present with a lack of symptoms commonly found in other mental disorders similar to paraphrenia. There is no significant deterioration of intellect, personality, or habits and patients often remain clean and mostly self-sufficient. Patients also remain oriented well in time and space.

Paraphrenia is different from schizophrenia because, while both disorders result in delusions and hallucinations, individuals with schizophrenia exhibit changes and deterioration of personality whereas individuals with paraphrenia maintain a well-preserved personality and affective response.

Delusional misidentification syndrome is an umbrella term, introduced by Christodoulou (in his book "The Delusional Misidentification Syndromes", Karger, Basel, 1986) for a group of delusional disorders that occur in the context of mental and neurological illness. They all involve a belief that the identity of a person, object, or place has somehow changed or has been altered. As these delusions typically only concern one particular topic, they also fall under the category called monothematic delusions.

This psychopathological syndrome is usually considered to include four main variants:

- The Capgras delusion is the belief that (usually) a close relative or spouse has been replaced by an identical-looking impostor.

- The Fregoli delusion is the belief that various people the believer meets are actually the same person in disguise.

- Intermetamorphosis is the belief that people in the environment swap identities with each other whilst maintaining the same appearance.

- Subjective doubles, described by Christodoulou in 1978 ("American Journal of Psychiatry" 135, 249, 1978), is the belief that there is a doppelgänger or double of him- or herself carrying out independent actions.

However, similar delusional beliefs, often singularly or more rarely reported, are sometimes also considered to be part of the delusional misidentification syndrome. For example:

- Mirrored-self misidentification is the belief that one's reflection in a mirror is some other person.

- Reduplicative paramnesia is the belief that a familiar person, place, object, or body part has been duplicated. For example, a person may believe that they are in fact not in the hospital to which they were admitted, but an identical-looking hospital in a different part of the country, despite this being obviously false.

- The Cotard delusion is a rare disorder in which people hold a delusional belief that they are dead (either figuratively or literally), do not exist, are putrefying, or have lost their blood or internal organs. In rare instances, it can include delusions of immortality.

- Syndrome of delusional companions is the belief that objects (such as soft toys) are sentient beings.

- Clonal pluralization of the self, where a person believes there are multiple copies of him- or herself, identical both physically and psychologically but physically separate and distinct.

There is considerable evidence that disorders such as the Capgras or Fregoli syndromes are associated with disorders of face perception and recognition. However, it has been suggested that all misidentification problems exist on a continuum of anomalies of familiarity, from déjà vu at one end to the formation of delusional beliefs at the other.

Secondary functional delusional parasitosis occurs when the delusions are associated with a psychiatric condition such as schizophrenia or clinical depression.

Individuals experiencing religious delusions are preoccupied with religious subjects that are not within the expected beliefs for an individual's background, including culture, education, and known experiences of religion. These preoccupations are incongruous with the mood of the subject. Falling within the definition also are delusions arising in psychotic depression; however, these must present within a major depressive episode and be congruous with mood.

Researchers in a 2000 study found religious delusions to be unrelated to any specific set of diagnostic criteria, but correlated with demographic criteria, primarily age. In a comparative study sampling 313 patients, those with religious delusion were found to be aged older, and had been placed on a drug regime or started a treatment programme at an earlier stage. In the context of presentation, their global functioning was found to be worse than another group of patients without religious delusions. The first group also scored higher on the Scale for the Assessment of Positive Symptoms (SAPS), had a greater total on the Brief Psychiatric Rating Scale (BPRS), and were treated with a higher mean number of neuroleptic medications of differing types during their hospitalization.

Religious delusion was found in 2007 to strongly correlate with "temporolimbic overactivity". This is a condition where irregularities in the brain's limbic system may present as symptoms of paranoid schizophrenia.

In a 2010 study, Swiss psychiatrists found religious delusions with themes of spiritual persecution by malevolent spirit-entities, control exerted over the person by spirit-entities, delusional experience of sin and guilt, or delusions of grandeur.

Religious delusions have generally been found to be less stressful than other types of delusion. A study found adherents to new religious movements to have similar delusionary cognition, as rated by the Delusions Inventory, to a psychotic group, although the former reported feeling less distressed by their experiences than the latter.

Persecutory delusions are a set of delusional conditions in which the affected persons believe they are being persecuted. Specifically, they have been defined as containing two central elements:

1. The individual thinks that harm is occurring, or is going to occur.

2. The individual thinks that the perceived persecutor has the intention to cause harm.

According to the DSM-IV-TR, persecutory delusions are the most common form of delusions in paranoid schizophrenia, where the person believes "he or she is being tormented, followed, tricked, spied on, or ridiculed." They are also often seen in schizoaffective disorder and, as recognized by DSM-IV-TR, constitute the cardinal feature of the persecutory subtype of delusional disorder, by far the most common. Delusions of persecution may also appear in manic and mixed episodes of bipolar disease, polysubstance abuse, and severe depressive episodes with psychotic features, particularly when associated with bipolar illness.

The delusion of negation is the central symptom in Cotard's syndrome. The patient afflicted with this mental illness usually denies his or her existence, the existence of a certain body part, or the existence of a portion of their body. Cotard's syndrome exists in three stages: (i) Germination stage—the symptoms of psychotic depression and of hypochondria appear; (ii) Blooming stage—the full development of the syndrome and the delusions of negation; and (iii) Chronic stage—continued, severe delusions along with chronic psychiatric depression.

The Cotard syndrome withdraws the afflicted person from other people due to the neglect of their personal hygiene and physical health. The delusion of negation of self prevents the patient from making sense of external reality, which then produces a distorted view of the external world. Such a delusion of negation is usually found in the psychotic patient who also presents with schizophrenia. Although a diagnosis of Cotard's syndrome does not require the patient's having had hallucinations, the strong delusions of negation are comparable to those found in schizophrenic patients.

Oneiroid syndrome, from the Ancient Greek "" ("oneiros", meaning "dream") and "" ("eidos", meaning "form, likeness"), is dream-like fantastic derangement of consciousness with illusions and hallucinations, catatonic symptoms and kaleidoscopic quality of psychopathological experiences. It's an element of the catatonic form of schizophrenia and presents with a dream-like or nightmare-like state as a background of intensive psychopathological experiences.

Oneiroid states were first described by the German physician Wilhelm Mayer-Gross in 1924. Mayer-Gross's 1924 habilitation on "Self-descriptions of Confusional States: the Oneiroid Form of Experience" () is considered the first monograph about oneiroid state. In this monograph the psychopathological method was used (German psychiatrists called that the "phenomenological method" – phänomenologische Methode).

The oneiroid syndrome, known to European and Russian psychiatrists, but all but forgotten in the USA.

Later in 1961 the Bulgarian psychiatrist S.T. Stoyanov studied the dynamics and the course of the oneiroid syndrome in "periodic", or recurrent schizophrenia (ICD-10).

According to this research the syndrome has six stages in its course:

1. initial general-somatic and vegetative disorder

2. delusional mood

3. affective-delusional depersonalisation and derealisation

4. fantastic-delusional and affective depersonalisation and derealisation

5. illusional depersonalisation and derealisation, and

6. catatonic-oneiroid state in the culmination.

In most of the cases of the oneiroid syndrome there were crude pathological changes in the electroencephalography (EEG).

The prognosis of oneiroid catatonia is optimal, in comparison with lucid catatonia.

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.

In primary delusional parasitosis, the delusions comprise the entire disease entity: there is no additional deterioration of basic mental functioning or idiosyncratic thought processes. The parasitic delusions consist of a single delusional belief regarding some aspect of health. This is also referred to as "monosymptomatic hypochondriacal psychosis", and sometimes as "true" delusional parasitosis. In the DSM-IV, this corresponds with "delusional disorder, somatic type".

The symptoms of the syndrome of subjective doubles are not clearly defined in medical literature, however, there are some defining features of the delusion:

- The existence of the delusion, by definition, is not a widely accepted cultural belief.

- The patient insists that the double he/she sees is real even when presented with contradictory evidence.

- Paranoia and/or spatial visualization ability impairments are present.

Similarities to other disorders are often noted in literature. Prosopagnosia, or the inability to recognize faces, may be related to this disorder due to the similarity of symptoms. Subjective doubles syndrome is also similar to delusional autoscopy, also known as an out-of-body experience, and therefore is occasionally referred to as an "autoscopic type" delusion. However, subjective doubles delusion differs from an autoscopic delusion: autoscopy often occurs during times of extreme stress, and can usually be treated by relieving the said stressor.

The syndrome of subjective doubles is usually accompanied by another mental disorder or organic brain syndrome, and may appear during or after the onset of the other disorder. Often, co-occurrence of subjective doubles with other types of delusional misidentification syndromes, especially Capgras syndrome, also occurs.

Several variations of the syndrome have been reported in literature:

- The doubles may appear at different ages of oneself.

- Some patients describe their double as both a physically and psychologically identical copy, rather than a purely physical copy. This is also known as clonal pluralization of the self, another type of delusional misidentification syndrome that may or may not be the same type of disorder (see #Controversy, below). In this case, depersonalization may be a symptom.

- Reverse subjective doubles occurs when the patient believes his/her own self (either physical or mental) is being transformed into another person. (see the case of Mr. A in #Presentation)

The article "Betwixt Life and Death: Case Studies of the Cotard Delusion" (1996) describes a contemporary case of Cotard delusion, which occurred in a Scotsman whose brain was damaged in a motorcycle accident:

The article "Recurrent Postictal Depression with Cotard Delusion" (2005) describes the case of a fourteen-year-old epileptic boy whose distorted perception of reality resulted from Cotard syndrome. His mental health history was of a boy expressing themes of death, chronic sadness, decreased physical activity in playtime, social withdrawal, and disturbed biological functions. About twice a year, the boy suffered episodes that lasted between three weeks and three months. In the course of each episode, he said that everyone and everything was dead (including trees), described himself as a dead body, and warned that the world would be destroyed within hours. Throughout the episode, the boy showed no response to pleasurable stimuli and had no interest in social activities.

"The following case describes a patient who was diagnosed with psychotic depression, bipolar disorder, and the syndrome of subjective doubles:"

Taken from Kamanitz et al., 1989:

"The following case describes a patient who has been diagnosed with schizoaffective disorder along with multiple delusional misidentification syndromes (subjective doubles, Capgras delusion, intermetamorphosis)":

Taken from Silva et al., 1994:

"The following case describes a patient who has been diagnosed with chronic paranoid schizophrenia and reverse subjective doubles:"

Taken from Vasavada and Masand, 1992:

Schizophrenia is often described in terms of positive and negative (or deficit) symptoms. "Positive symptoms" are those that most individuals do not normally experience, but are present in people with schizophrenia. They can include delusions, disordered thoughts and speech, and tactile, auditory, visual, olfactory and gustatory hallucinations, typically regarded as manifestations of psychosis. Hallucinations are also typically related to the content of the delusional theme. Positive symptoms generally respond well to medication.

"Negative symptoms" are deficits of normal emotional responses or of other thought processes, and are less responsive to medication. They commonly include flat expressions or little emotion, poverty of speech, inability to experience pleasure, lack of desire to form relationships, and lack of motivation. Negative symptoms appear to contribute more to poor quality of life, functional ability, and the burden on others than positive symptoms do. People with greater negative symptoms often have a history of poor adjustment before the onset of illness, and response to medication is often limited.

The validity of the positive and negative construct has been challenged by factor analysis studies observing a three dimension grouping of symptoms. While different terminology is used, a dimension for hallucinations, a dimension for disorganization, and a dimension for negative symptoms are usually described.