Formal diagnosis may be performed by a psychologist, psychiatrist, clinical social worker, or other licensed mental health professional. To be diagnosed with OCD, a person must have obsessions, compulsions, or both, according to the Diagnostic and Statistical Manual of Mental Disorders (DSM). The Quick Reference to the 2000 edition of the DSM states that several features characterize clinically significant obsessions and compulsions. Such obsessions, the DSM says, are recurrent and persistent thoughts, impulses or images that are experienced as intrusive and that cause marked anxiety or distress. These thoughts, impulses or images are of a degree or type that lies outside the normal range of worries about conventional problems. A person may attempt to ignore or suppress such obsessions, or to neutralize them with some other thought or action, and will tend to recognize the obsessions as idiosyncratic or irrational.

Compulsions become clinically significant when a person feels driven to perform them in response to an obsession, or according to rules that must be applied rigidly, and when the person consequently feels or causes significant distress. Therefore, while many people who do not suffer from OCD may perform actions often associated with OCD (such as ordering items in a pantry by height), the distinction with clinically significant OCD lies in the fact that the person who suffers from OCD "must" perform these actions, otherwise they will experience significant psychological distress. These behaviors or mental acts are aimed at preventing or reducing distress or preventing some dreaded event or situation; however, these activities are not logically or practically connected to the issue, or they are excessive. In addition, at some point during the course of the disorder, the individual must realize that their obsessions or compulsions are unreasonable or excessive.

Moreover, the obsessions or compulsions must be time-consuming (taking up more than one hour per day) or cause impairment in social, occupational or scholastic functioning. It is helpful to quantify the severity of symptoms and impairment before and during treatment for OCD. In addition to the peron's estimate of the time spent each day harboring obsessive-compulsive thoughts or behaviors, concrete tools can be used to gauge the people’s condition. This may be done with rating scales, such as the Yale–Brown Obsessive Compulsive Scale (Y-BOCS). With measurements like these, psychiatric consultation can be more appropriately determined because it has been standardized.

OCD is sometimes placed in a group of disorders called the obsessive–compulsive spectrum.

OCD is often confused with the separate condition obsessive–compulsive personality disorder (OCPD). OCD is egodystonic, meaning that the disorder is incompatible with the sufferer's self-concept. Because ego dystonic disorders go against a person's self-concept, they tend to cause much distress. OCPD, on the other hand, is egosyntonic—marked by the person's acceptance that the characteristics and behaviours displayed as a result are compatible with their self-image, or are otherwise appropriate, correct or reasonable.

As a result, people with OCD are often aware that their behavior is not rational, are unhappy about their obsessions but nevertheless feel compelled by them. By contrast people with OCPD are not aware of anything abnormal; they will readily explain why their actions are rational, it is usually impossible to convince them otherwise, and they tend to derive pleasure from their obsessions or compulsions.

Theodore Millon identified five subtypes of the compulsive personality (2004). Any compulsive personality may exhibit one or more of the following:

Not only are there significant health risks associated with compulsive hoarding, but scientists are also trying to pinpoint how significant the interference is with occupational and social functioning in a hoarder's daily life. In a pool of compulsive hoarders, 42% found their behavior problematic to the 63% of their family and friends who saw the behavior as problematic. The findings suggest that individuals who hoard may exhibit impaired sensitivity to their own and others’ emotions, and conversely, relate the world around them by forming attachments to possessions rather than to people. Lower emotional intelligence among hoarding patients may also impact their ability to discard and organize their possessions. With such detrimental characteristics, comprehensive research has been performed to find a cure. Although this is ongoing research, most investigations have found that only a third of patients who hoard show an adequate response to these medications and therapeutic interventions.

With the modifications to the DSM, insurance coverage for treatments will change as well as special education programs.

Impulse-control disorders have two treatment options: psychosocial and pharmacological. Treatment methodology is informed by the presence of comorbid conditions.

Treatment for OCPD includes psychotherapy, cognitive behavioral therapy, behavior therapy or self-help. Medication may be prescribed. In behavior therapy, a person with OCPD discusses with a psychotherapist ways of changing compulsions into healthier, productive behaviors. Cognitive analytic therapy is an effective form of behavior therapy.

Treatment is complicated if the person does not accept that they have OCPD, or believes that their thoughts or behaviors are in some sense correct and therefore should not be changed. Medication alone is generally not indicated for this personality disorder. Selective serotonin reuptake inhibitors (SSRIs) may be useful in addition to psychotherapy by helping the person with OCPD be less bogged down by minor details, and to lessen how rigid they are.

People with OCPD are three times more likely to receive individual psychotherapy than people with major depressive disorder. There are higher rates of primary care utilization. There are no known properly controlled studies of treatment options for OCPD. More research is needed to explore better treatment options.

Complications of late Parkinson's disease may include a range of impulse-control disorders, including eating, buying, compulsive gambling and sexual behavior. One study found that 13.6% of Parkinson's patients exhibited at least one form of ICD. There is a significant co-occurrence of pathological gambling and personality disorder, and is suggested to be caused partly by their common "genetic vulnerability". The degree of heritability to ICD is similar to other psychiatric disorders including substance abuse disorder. There has also been found a genetic factor to the development of ICD just as there is for substance abuse disorder. The risk for subclinical PG in a population is accounted for by the risk of alcohol dependence by about 12-20% genetic and 3-8% environmental factors. There is a high rate of comorbidity between ADHD and other impulse-control disorders.

Education, and a "watch and wait" strategy, are the only treatment needed for many, and the majority of individuals with tics do not seek treatment; treatment of tic disorders is similar to treatment of Tourette syndrome.

Research on treating BDD is limited. Yet anti-depressant medication, such as selective serotonin reuptake inhibitors (SSRIs), and cognitive-behavioral therapy (CBT) are considered effective. SSRIs can help relieve obsessive-compulsive and delusional traits, while cognitive-behavioral therapy can help patients recognize faulty thought patterns. Before treatment, it can help to provide psychoeducation, as with self-help books and support websites.

Cognitive-behavioral therapy (CBT) is a commonly implemented therapeutic intervention for compulsive hoarding. As part of cognitive behavior therapy, the therapist may help the patient to:

- Discover why he or she is compelled to hoard.

- Learn to organize possessions in order to decide what to discard.

- Develop decision-making skills.

- Declutter the home during in-home visits by a therapist or professional organizer.

- Gain and perform relaxation skills.

- Attend family and/or group therapy.

- Be open to trying psychiatric hospitalization if the hoarding is serious.

- Have periodic visits and consultations to keep a healthy lifestyle.

This modality of treatment usually involves exposure and response prevention to situations that cause anxiety and cognitive restructuring of beliefs related to hoarding. Furthermore, research has also shown that certain CBT protocols have been more effective in treatment than others. CBT programs that specifically address the motivation of the sufferer, organization, acquiring new clutter, and removing current clutter from the home have shown promising results. This type of treatment typically involves in-home work with a therapist combined with between-session homework, the completion of which is associated with better treatment outcomes. Research on Internet-based CBT treatments for the disorder (where participants have access to educational resources, cognitive strategies, and chat groups) has also shown promising results both in terms of short- and long-term recovery.

Other therapeutic approaches that have been found to be helpful are:

1. Motivational interviewing: originated in addiction therapy. This method is significantly helpful when used in hoarding cases in which insight is poor and ambivalence around change is marked.

2. Harm reduction rather than symptom reduction: also borrowed from addiction therapy. The goal is to decrease the harmful implications of the behavior, rather than the hoarding behaviors.

3. Group therapy: reduces social isolation and social anxiety and is cost-effective compared to one-on-one intervention.

4. Eye movement desensitization and reprocessing (EMDR) has been employed, although there is insufficient evidence for EMDR to be considered effective for treating compulsive hoarding (as for treating obsessive-compulsive disorders in general).

Individuals with hoarding behaviors are often described as having low motivation and poor compliance levels, and as being indecisive and procrastinators, which may frequently lead to premature termination (i.e., dropout) or low response to treatment. Therefore, it was suggested that future treatment approaches, and pharmacotherapy in particular, be directed to address the underlying mechanisms of cognitive impairments demonstrated by individuals with hoarding symptoms.

Mental health professionals frequently express frustration regarding hoarding cases, mostly due to premature termination and poor response to treatment. Patients are frequently described as indecisive, procrastinators, recalcitrant, and as having low or no motivation, which can explain why many interventions fail to accomplish significant results. To overcome this obstacle, some clinicians recommend accompanying individual therapy with home visits to help the clinician:

Likewise, certain cases are assisted by professional organizers as well.

Estimates of prevalence and gender distribution have varied widely via discrepancies in diagnosis and reporting. In American psychiatry, BDD gained diagnostic criteria in the "DSM-IV", but clinicians' knowledge of it, especially among general practitioners, is constricted. Meanwhile, shame about having the bodily concern, and fear of the stigma of vanity, makes many hide even having the concern.

Via shared symptoms, BDD is commonly misdiagnosed as social-anxiety disorder, obsessive-compulsive disorder, major depressive disorder, or social phobia. Correct diagnosis can depend on specialized questioning and correlation with emotional distress or social dysfunction. Estimates place the Body Dysmorphic Disorder Questionnaire's sensitivity at 100% (0% false negatives) and specificity at 92.5% (7.5% false positives).

Different assessment tools can be used to determine if an individual is addicted to exercise. Most tools used to determine risk for exercise addiction are modified tools that have been used for assessing other behavioral addictions. Tools for determining eating disorders can also show a high risk for exercise addiction.

The Obligatory Exercise Questionnaire was created by Thompson and Pasman in 1991, consisting of 20 questions on exercise habits and attitudes toward exercise and body image. Patients respond to statements on a scale of 1 (never) to 4 (always). This questionnaire aided in the development of another assessment tool, the Exercise Addiction Inventory.

The Exercise Addiction Inventory was developed by Terry "et al" in 2004. This inventory was developed as a self-report to examine an individual's beliefs toward exercise. The inventory is made up of six statements in relation to the perception of exercise, concerning: the importance of exercise to the individual, relationship conflicts due to exercise, how mood changes with exercise, the amount of time spent exercising, the outcome of missing a workout, and the effects of decreasing physical activity. Individuals are asked to rate each statement from 1 (strongly disagree) to 5 (strongly agree). If an individual scores above 24 they are said to be at-risk for exercise addiction.

There has been controversy over the creation of a separate category in the DSM-5 for excoriation (skin picking) disorder. Two of the main reasons for objecting to the inclusion of excoriation disorder in the DSM-5 are: (1) that excoriation disorder may just be a symptom of a different underlying disorder, e.g. OCD or BDD, and (2) that excoriation disorder is merely a bad habit and that by allowing this disorder to obtain its own separate category it would force the DSM to include a wide array of bad habits as separate syndromes, e.g., nail biting and nose-picking. Stein has argued that excoriation disorder does qualify as a separate syndrome and should be classified as its own category because: (1) excoriation disorder occurs as the primary disorder and not as a subset of a larger disorder; (2) excoriation disorder has well-defined clinical features; (3) there is gathering data on the clinical features and diagnostic criteria for this condition; (4) there is sufficient data to create this as a separate category for excoriation disorder; (5) the incidence rate for excoriation disorder is high within the population; (6) diagnostic criteria for the disease have already been proposed; (7) the classification of excoriation disorder as a separate condition would lead to better studies and better treatment outcomes; and (8) classification as a separate condition would lead to more awareness of the disorder and encourage more people to obtain treatment.

Because excoriation disorder is different from other conditions and disorders that cause picking of the skin, it is important that any diagnosis of excoriation disorder take into account various other medical conditions as possible causes before diagnosing the patient with excoriation disorder. There are a variety of conditions that cause itching and skin picking including: eczema, psoriasis, diabetes, liver disease, Hodgkin's disease, polycythemia vera, systemic lupus, and Prader-Willi syndrome.

In order to better understand excoriation disorder, researchers have developed a variety of scales to categorize skin-picking behavior. These include the Skin-Picking Impact Scale (SPIS), and The Milwaukee Inventory for the Dimensions of Adult Skin-picking. The SPIS was created to measure how skin picking affects the individual socially, behaviorally, and emotionally.

As of the release of the fifth Diagnostic and Statistical Manual of Mental Disorders in May 2013, this disorder is classified as its own separate condition under "Obsessive Compulsive and Related Disorders" and is termed "excoriation (skin-picking) disorder".

The most common instrument used to screen for "probable pathological gambling" behavior is the South Oaks Gambling Screen (SOGS) developed by Lesieur and Blume (1987) at the South Oaks Hospital in New York City. In recent years the use of SOGS has declined due to a number of criticisms, including that it overestimates false positives (Battersby, Tolchard, Thomas & Esterman, 2002).

The "DSM-IV" diagnostic criteria presented as a checklist is an alternative to SOGS, it focuses on the psychological motivations underpinning problem gambling and was developed by the American Psychiatric Association. It consists of ten diagnostic criteria. One frequently used screening measure based upon the DSM-IV criteria is the National Opinion Research Center DSM Screen for Gambling Problems (NODS). The Canadian Problem Gambling Inventory (CPGI) and the Victorian Gambling Screen (VGS) are newer assessment measures. The Problem Gambling Severity Index, which focuses on the harms associated with problem gambling, is composed of nine items from the longer CPGI. The VGS is also harm based and includes 15 items. The VGS has proven validity and reliability in population studies as well as Adolescents and clinic gamblers.

The following conditions have been hypothesized by various researchers as existing on the spectrum.

However, recently there is a growing support for proposals to narrow down this spectrum to only include body dysmorphic disorder, hypochondriasis, tic disorders, and trichotillomania.

The obsessive–compulsive spectrum is a model of medical classification where various psychiatric, neurological and/or medical conditions are described as existing on a spectrum of conditions related to obsessive–compulsive disorder (OCD). "The disorders are thought to lie on a spectrum from impulsive to compulsive where impulsivity is said to persist due to deficits in the ability to inhibit repetitive behavior with known negative consequences, while compulsivity persists as a consequence of deficits in recognizing completion of tasks." OCD is a mental disorder characterized by obsessions and/or compulsions. An obsession is defined as "a recurring thought, image, or urge that the individual cannot control". Compulsion can be described as a "ritualistic behavior that the person feels compelled to perform". The model suggests that many conditions overlap with OCD in symptomatic profile, demographics, family history, neurobiology, comorbidity, clinical course and response to various pharmacotherapies. Conditions described as being on the spectrum are sometimes referred to as "obsessive–compulsive spectrum disorders".

A 2007 study found that 78% of a clinical sample of OCD patients had intrusive images. Most people who suffer from intrusive thoughts have not identified themselves as having OCD, because they may not have what they believe to be classic symptoms of OCD, such as handwashing. Yet, epidemiological studies suggest that intrusive thoughts are the most common kind of OCD worldwide; if people in the United States with intrusive thoughts gathered, they would form the fourth-largest city in the US, following New York City, Los Angeles, and Chicago.

The prevalence of OCD in every culture studied is at least 2% of the population, and the majority of those have obsessions, or bad thoughts, only; this results in a conservative estimate of more than 2 million sufferers in the United States alone (as of 2000). One author estimates that one in 50 adults have OCD and about 10–20% of these have sexual obsessions. A recent study found that 25% of 293 patients with a primary diagnosis of OCD had a history of sexual obsessions.

Antidepressants or antipsychotic medications may be used for more severe cases if intrusive thoughts do not respond to cognitive behavioral or exposure therapy alone. Whether the cause of intrusive thoughts is OCD, depression, or post-traumatic stress disorder, the selective serotonin reuptake inhibitor (SSRI) drugs (a class of antidepressants) are the most commonly prescribed. Intrusive thoughts may occur in persons with Tourette syndrome (TS) who also have OCD; the obsessions in TS-related OCD are thought to respond to SSRI drugs as well.

Antidepressants which have been shown to be effective in treating OCD include fluvoxamine (trade name Luvox), fluoxetine (Prozac), sertraline (Zoloft), paroxetine (Paxil), citalopram (Celexa), and clomipramine (Anafranil). Although SSRIs are known to be effective for OCD in general, there have been fewer studies on their effectiveness for intrusive thoughts. A retrospective chart review of patients with sexual symptoms treated with SSRIs showed the greatest improvement was in those with intrusive sexual obsessions typical of OCD. A study of ten patients with religious or blasphemous obsessions found that most patients responded to treatment with fluoxetine or clomipramine. Women with postpartum depression often have anxiety as well, and may need lower starting doses of SSRIs; they may not respond fully to the medication, and may benefit from adding cognitive behavioral or response prevention therapy.

Patients with intense intrusive thoughts that do not respond to SSRIs or other antidepressants may be prescribed typical and atypical neuroleptics including risperidone (trade name Risperdal), ziprasidone (Geodon), haloperidol (Haldol), and pimozide (Orap).

Studies suggest that therapeutic doses of inositol may be useful in the treatment of obsessive thoughts.

This diagnostic rubric is not recommended for general use because it is not clearly demarcated either from simple schizophrenia or from schizoid or paranoid personality disorders, or possibly autism and Asperger syndrome as currently diagnosed. If the term is used, three or four of the typical features listed above should have been present, continuously or episodically, for at least 2 years. The individual must never have met criteria for schizophrenia itself. A history of schizophrenia in a first-degree relative gives additional weight to the diagnosis but is not a prerequisite.

Typically, individuals with excoriation disorder find that the disorder interferes with daily life. Hindered by shame, embarrassment, and humiliation, they may take measures to hide their disorder by not leaving home, wearing long sleeves and pants even in heat, or covering visible damage to skin with cosmetics and/or bandages. Activities such as typing may be painful for those who pick at their fingers or hands, or walking for those who pick at the soles of their feet.

Some treatments for internalizing disorders include antidepressants, electroconvulsive therapy, and psychotherapy.

According to Theodore Millon, the schizotypal is one of the easiest personality disorders to identify but one of the most difficult to treat with psychotherapy. Persons with STPD usually consider themselves to be simply eccentric, productive, or nonconformist. As a rule, they underestimate maladaptiveness of their social isolation and perceptual distortions. It is not so easy to gain rapport with people who suffer from STPD due to the fact that increasing familiarity and intimacy usually increase their level of anxiety and discomfort. In most cases they do not respond to informality and humor.

Group therapy is recommended for persons with STPD only if the group is well structured and supportive. Otherwise, it could lead to loose and tangential ideation. Support is especially important for schizotypal patients with predominant paranoid symptoms, because they will have a lot of difficulties even in highly structured groups.

Tics should be distinguished from other causes of tourettism, stereotypies, chorea, dyskinesias, myoclonus, and obsessive-compulsive disorder.

Compulsive talking goes beyond the bounds of what is considered to be a socially acceptable amount of talking. The two main factors in determining if someone is a compulsive talker are talking in a continuous manner, only stopping when the other person starts talking, and others perceiving their talking as a problem. Personality traits that have been positively linked to this compulsion include assertiveness, willingness to communicate, self-perceived communication competence, and neuroticism. Studies have shown that most people who are talkaholics are aware of the amount of talking they do, are unable to stop, and do not see it as a problem.

An internalizing disorder is one type of emotional and behavioral disorder, along with externalizing disorders, and low incidence disorders. One who suffers from an internalizing disorder will keep their problems to themselves, or internalize the problems.