Psychiatrists seek to provide a medical diagnosis of individuals by an assessment of symptoms, signs and impairment associated with particular types of mental disorder. Other mental health professionals, such as clinical psychologists, may or may not apply the same diagnostic categories to their clinical formulation of a client's difficulties and circumstances. The majority of mental health problems are, at least initially, assessed and treated by family physicians (in the UK general practitioners) during consultations, who may refer a patient on for more specialist diagnosis in acute or chronic cases.

Routine diagnostic practice in mental health services typically involves an interview known as a mental status examination, where evaluations are made of appearance and behavior, self-reported symptoms, mental health history, and current life circumstances. The views of other professionals, relatives or other third parties may be taken into account. A physical examination to check for ill health or the effects of medications or other drugs may be conducted. Psychological testing is sometimes used via paper-and-pen or computerized questionnaires, which may include algorithms based on ticking off standardized diagnostic criteria, and in rare specialist cases neuroimaging tests may be requested, but such methods are more commonly found in research studies than routine clinical practice.

Time and budgetary constraints often limit practicing psychiatrists from conducting more thorough diagnostic evaluations. It has been found that most clinicians evaluate patients using an unstructured, open-ended approach, with limited training in evidence-based assessment methods, and that inaccurate diagnosis may be common in routine practice. In addition, comorbidity is very common in psychiatric diagnosis, where the same person meets the criteria for more than one disorder. On the other hand, a person may have several different difficulties only some of which meet the criteria for being diagnosed. There may be specific problems with accurate diagnosis in developing countries.

More structured approaches are being increasingly used to measure levels of mental illness.

- HoNOS is the most widely used measure in English mental health services, being used by at least 61 trusts. In HoNOS a score of 0–4 is given for each of 12 factors, based on functional living capacity. Research has been supportive of HoNOS, although some questions have been asked about whether it provides adequate coverage of the range and complexity of mental illness problems, and whether the fact that often only 3 of the 12 scales vary over time gives enough subtlety to accurately measure outcomes of treatment.

Conduct disorder is classified in the fourth edition of "Diagnostic and Statistical Manual of Mental Disorders" (DSM). It is diagnosed based on a prolonged pattern of antisocial behaviour such as serious violation of laws and social norms and rules in people younger than the age of 18. Similar criteria are used in those over the age of 18 for the diagnosis of antisocial personality disorder. No proposed revisions for the main criteria of conduct disorder exist in the "DSM-5"; there is a recommendation by the work group to add an additional specifier for callous and unemotional traits. According to DSM-5 criteria for conduct disorder, there are four categories that could be present in the child's behavior: aggression to people and animals, destruction of property, deceitfulness or theft, and serious violation of rules.

Almost all adolescents who have a substance use disorder have conduct disorder-like traits, but after successful treatment of the substance use disorder, about half of these adolescents no longer display conduct disorder-like symptoms. Therefore, it is important to exclude a substance-induced cause and instead address the substance use disorder prior to making a psychiatric diagnosis of conduct disorder.

About 25-40% of youths diagnosed with conduct disorder qualify for a diagnosis of antisocial personality disorder when they reach adulthood. For those that do not develop ASPD, most still exhibit social dysfunction in adult life.

For a child or adolescent to qualify for a diagnosis of ODD, behaviours must cause considerable distress for the family or interfere significantly with academic or social functioning. Interference might take the form of preventing the child or adolescent from learning at school or making friends, or placing him or her in harmful situations. These behaviours must also persist for at least six months. Effects of ODD can be greatly amplified by other disorders in comorbidity such as ADHD. Other common comorbid disorders include depression and substance use disorders.

Since the 1980s, Paula Caplan has been concerned about the subjectivity of psychiatric diagnosis, and people being arbitrarily “slapped with a psychiatric label.” Caplan says because psychiatric diagnosis is unregulated, doctors are not required to spend much time interviewing patients or to seek a second opinion. The Diagnostic and Statistical Manual of Mental Disorders can lead a psychiatrist to focus on narrow checklists of symptoms, with little consideration of what is actually causing the patient’s problems. So, according to Caplan, getting a psychiatric diagnosis and label often stands in the way of recovery.

In 2013, psychiatrist Allen Frances wrote a paper entitled "The New Crisis of Confidence in Psychiatric Diagnosis", which said that "psychiatric diagnosis… still relies exclusively on fallible subjective judgments rather than objective biological tests." Frances was also concerned about "unpredictable overdiagnosis." For many years, marginalized psychiatrists (such as Peter Breggin, Thomas Szasz) and outside critics (such as Stuart A. Kirk) have "been accusing psychiatry of engaging in the systematic medicalization of normality." More recently these concerns have come from insiders who have worked for and promoted the American Psychiatric Association (e.g., Robert Spitzer, Allen Frances). A 2002 editorial in the "British Medical Journal" warned of inappropriate medicalization leading to disease mongering, where the boundaries of the definition of illnesses are expanded to include personal problems as medical problems or risks of diseases are emphasized to broaden the market for medications.

SCT is currently not an official diagnosis in DSM-5. But there are rating scales that can be used to screen for SCT symptoms such as the "Concentration Inventory" (for children and adults) or the "Barkley Sluggish Cognitive Tempo Scale-Children and Adolescents (BSCTS-CA)". The "Comprehensive Behaviour Rating Scale for Children" (CBRSC), an older scale, can also be used for SCT as this case study shows.

Although having no diagnosic code either, ICD-10 mentions the SCT group as a reason for why it did not replace the term ""Hyperkinetic Disorder"" with ""ADHD"."

Other mental disorders may produce similar symptoms to SCT (e.g. excessive daydreaming or "staring blankly") and should not be confused with it. Examples might be conditions like depersonalization disorder, dysthymia, thyroid problems, absence seizures, Bipolar II disorder, Kleine–Levin syndrome, forms of autism or schizoid personality disorder. However, the prevalence of SCT in these clinical populations has yet to be empirically and systematically investigated.

In the absence of a standardized diagnosis system, many popular, informal classification systems or checklists, outside the DSM and ICD, were created out of clinical and parental experience within the field known as attachment therapy. These lists are unvalidated and critics state they are inaccurate, too broadly defined or applied by unqualified persons. Many are found on the websites of attachment therapists. Common elements of these lists such as lying, lack of remorse or conscience and cruelty do not form part of the diagnostic criteria under either DSM-IV-TR or ICD-10. Many children are being diagnosed with RAD because of behavioral problems that are outside the criteria. There is an emphasis within attachment therapy on aggressive behavior as a symptom of what they describe as attachment disorder whereas mainstream theorists view these behaviors as comorbid, externalizing behaviors requiring appropriate assessment and treatment rather than attachment disorders. However, knowledge of attachment relationships can contribute to the cause, maintenance and treatment of externalizing disorders.

The Randolph Attachment Disorder Questionnaire or RADQ is one of the better known of these checklists and is used by attachment therapists and others. The checklist includes 93 discrete behaviours, many of which either overlap with other disorders, like conduct disorder and oppositional defiant disorder, or are not related to attachment difficulties. Critics assert that it is unvalidated and lacks specificity.

The diagnostic complexities of RAD mean that careful diagnostic evaluation by a trained mental health expert with particular expertise in differential diagnosis is considered essential. Several other disorders, such as conduct disorders, oppositional defiant disorder, anxiety disorders, post traumatic stress disorder and social phobia share many symptoms and are often comorbid with or confused with RAD, leading to over and under diagnosis. RAD can also be confused with neuropsychiatric disorders such as autism, pervasive developmental disorder, childhood schizophrenia and some genetic syndromes. Infants with this disorder can be distinguished from those with organic illness by their rapid physical improvement after hospitalization. Autistic children are likely to be of normal size and weight and often exhibit a degree of intellectual disability. They are unlikely to improve upon being removed from the home.

There is a considerable personality disorder diagnostic co-occurrence. Patients who meet the DSM-IV-TR diagnostic criteria for one personality disorder are likely to meet the diagnostic criteria for another. Diagnostic categories provide clear, vivid descriptions of discrete personality types but the personality structure of actual patients might be more accurately described by a constellation of maladaptive personality traits.

"Sites used DSM-III-R criterion sets. Data obtained for purposes of informing the development of the DSM-IV-TR personality disorder diagnostic criteria."

Abbreviations used: "PPD – Paranoid Personality Disorder, SzPD – Schizoid Personality Disorder, StPD – Schizotypal Personality Disorder, ASPD – Antisocial Personality Disorder, BPD – Borderline Personality Disorder, HPD – Histrionic Personality Disorder, NPD – Narcissistic Personality Disorder, AvPD – Avoidant Personality Disorder, DPD – Dependent Personality Disorder, OCPD – Obsessive-Compulsive Personality Disorder, PAPD – Passive-Aggressive Personality Disorder."

Despite recent initiatives to study psychopathology along dimensions of behavior and neurobiological indices, which would help refine a clearer picture of the development and treatment of externalizing disorders, the majority of research has examined specific mental disorders. Thus, best practices for many externalizing disorders are disorder-specific. For example, substance use disorders themselves are very heterogeneous and their best-evidenced treatment typically includes cognitive behavioral therapy, motivational interviewing, and a substance disorder-specific detoxification or psychotropic medication treatment component. The best-evidenced treatment for childhood conduct and externalizing problems more broadly, including youth with ADHD, ODD, and CD, is parent management training, a form of cognitive behavioral therapy. Additionally, individuals with ADHD, both youth and adults, are frequently treated with stimulant medications (or alternative psychotropic medications), especially if psychotherapy alone has not been effective in managing symptoms and impairment. Psychotherapy and medication interventions for individuals with severe, adult forms of antisocial behavior, such as antisocial personality disorder, have been mostly ineffective. An individual's comorbid psychopathology may also influences the course of treatment for an individual.

The prognosis of SCT is unknown. In contrast, much is known about the adolescent and adult outcomes of children having ADHD. Those with SCT symptoms typically show a later onset of their symptoms than do those with ADHD, perhaps by as much as a year or two later on average. They have as much or more difficulty with academic tasks and far fewer social difficulties than do people having ADHD. They do not have the same risks for oppositional defiant disorder, conduct disorder, or social aggression and thus may have different life course outcomes compared to children with ADHD-HI and Combined subtypes who have far higher risks for these other "externalizing" disorders.

However, unlike ADHD, there are no longitudinal studies of children with SCT that can shed light on the developmental course and adolescent or adult outcomes of these individuals.

Currently, genetic research for the understanding of the development of personality disorders is severely lacking. However, there are a few possible risk factors currently in discovery. Researchers are currently looking into genetic mechanisms for traits such as aggression, fear and anxiety, which are associated with diagnosed individuals. More research is being conducted into disorder specific mechanisms.

Approaches to the treatment of ODD include parent management training, individual psychotherapy, family therapy, cognitive behavioral therapy, and social skills training. According to the American Academy of Child and Adolescent Psychiatry, treatments for ODD are tailored specifically to the individual child, and different treatment techniques are applied for pre-schoolers and adolescents. Several preventative programs have had a positive effect on those at high risk for ODD. Both home visitation and programs such as Head Start have shown some effectiveness in preschool children. Social skills training, parent management training, and anger management programs have been used as prevention programs for school-age children at risk for ODD. For adolescents at risk for ODD, cognitive interventions, vocational training and academic tutoring have shown preventative effectiveness. There is also limited evidence that the atypical antipsychotic medication risperidone decreases aggression and conduct problems in youth with disruptive behavioral disorders, such as ODD.

ADHD often precedes the onset of ODD, and approximately half of children with ADHD, Combined Type also have ODD. ODD is a risk factor for CD and frequently precedes the onset of CD symptoms. Children with an early onset of CD symptoms, with at least one symptom before age 10 years, are at risk for more severe and persistent antisocial behavior continuing into adulthood. Youth with early-onset conduct problems are particularly at risk for ASPD (note that an onset of CD prior to age 15 is part of the diagnostic criteria for ASPD), whereas CD is typically limited to adolescence when youth's CD symptoms begin during adolescence.

Similar to adult personality disorders there are multiple causes and causal interactions for personality development disorders. In clinical practice it is important to view the disorder multi-perspectively and from an individual perspective. Biological and neurological causes need to be observed just as much as psychosocial factors. Looking at the disorder from only one perspective (e.g. (s)he had a bad childhood) often results in ignorance of important other factors or causal interactions. This might be one of the main reasons why traditional treatment methods often fail with these disorders. Only a multi-perspective view can provide for a multi-dimensional treatment approach which seems to be the key for these disorders.

The diagnosis personality development disorder should only be given carefully and after a longer period of evaluation. Also a thorough diagnostic evaluation is necessary. Parents should be questioned separately and together with the child or adolescent to evaluate the severity and duration of the problems. In addition standardized personality tests might be helpful. It is also useful to ask the family what treatment approaches they have already tried so far without success.

Personality development disorders usually need a complex and multi-dimensional treatment approach (Adam & Breithaupt-Peters, 2010). Since the problems are complex, treatment needs to affect the conditions in all impaired functional and social areas. Both educational and therapeutic methods are helpful and problem and strength based approaches work hand in hand. Parents need to be included as well as the school environment. Treatment methods need to be flexible and adjustable to the individual situation. Even elements of social work can be helpful when supporting the families and in some cases medication might be necessary. When suicidal behaviors or self-injuries are prominent treatment might best be done in a hospital.

For some personality development disorders (e.g. borderline personality disorder) treatment methods from adults can be adapted (e.g. dialectical behavior therapy, Miller et a., 2006).

In the opinion of Allen Frances, chair of the DSM-IV task force, the DSM-5's somatic symptom disorder brings with it a risk of mislabeling a sizable proportion of the population as mentally ill. “Millions of people could be mislabeled, with the burden falling disproportionately on women, because they are more likely to be casually dismissed as ‘catastrophizers’ when presenting with physical symptoms.”

The PCL-R, the PCL:SV, and the PCL:YV are highly regarded and widely used in criminal justice settings, particularly in North America. They may be used for risk assessment and for assessing treatment potential and be used as part of the decisions regarding bail, sentence, which prison to use, parole, and regarding whether a youth should be tried as a juvenile or as an adult. There have been several criticisms against its use in legal settings. They include the general criticisms against the PCL-R, the availability of other risk assessment tools which may have advantages, and the excessive pessimism surrounding the prognosis and treatment possibilities of those who are diagnosed with psychopathy.

The interrater reliability of the PCL-R can be high when used carefully in research but tend to be poor in applied settings. In particular Factor 1 items are somewhat subjective. In sexually violent predator cases the PCL-R scores given by prosecution experts were consistently higher than those given by defense experts in one study. The scoring may also be influenced by other differences between raters. In one study it was estimated that of the PCL-R variance, about 45% was due to true offender differences, 20% was due to which side the rater testified for, and 30% was due to other rater differences.

To aid a criminal investigation, certain interrogation approaches may be used to exploit and leverage the personality traits of suspects thought to have psychopathy and make them more likely to divulge information.

Somatic symptom disorder has been a controversial diagnosis, since it was historically based primarily on negative criteria - that is, the absence of a medical explanation for the presenting physical complaints. Consequently, any person suffering from a poorly understood illness can potentially fulfill the criteria for this psychiatric diagnosis, even if they exhibit no psychiatric symptoms in the conventional sense. In 2013-4, there were several widely publicized cases of individuals being involuntarily admitted to psychiatric wards on the basis of this diagnosis alone. This has raised concerns about the consequences of potential misuse of this diagnostic category.

The lifetime prevalence of dissociative disorders varies from 10% in the general population to 46% in psychiatric inpatients. Diagnosis can be made with the help of structured interviews such as the Dissociative Disorders Interview Schedule (DDIS) and the Structured Clinical Interview for DSM-IV Dissociative Disorders (SCID-D), or with the Dissociative Experiences Scale (DES) which is a self-assessment questionnaire. Some diagnostic tests have also been adapted and/or developed for use with children and adolescents such as the Children's Version of the Response Evaluation Measure (REM-Y-71), Child Interview for Subjective Dissociative Experiences, Child Dissociative Checklist (CDC), Child Behavior Checklist (CBCL) Dissociation Subscale, and the Trauma Symptom Checklist for Children Dissociation Subscale.

There are problems with classification, diagnosis and therapeutic strategies of dissociative and conversion disorders which can be understood by the historic context of hysteria. Even current systems used to diagnose DD such as the DSM-IV and ICD-10 differ in the way the classification is determined. In most cases mental health professionals are still hesitant to diagnose patients with Dissociative Disorder, because before they are considered to be diagnosed with Dissociative Disorder these patients have more than likely been diagnosed with major depression, anxiety disorder, and most often post-traumatic disorder.

An important concern in the diagnosis of dissociative disorders is the possibility that the patient may be feigning symptoms in order to escape negative consequences. Young criminal offenders report much higher levels of dissociative disorders, such as amnesia. In one study it was found that 1% of young offenders reported complete amnesia for a violent crime, while 19% claimed partial amnesia. There have also been incidences in which people with dissociative identity disorder provide conflicting testimonies in court, depending on the personality that is present.

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.

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

Dissociative disorders (DD) are widely believed to have roots in traumatic childhood experience (abuse or loss), but symptomology often goes unrecognized or is misdiagnosed in children and adolescents. There are several reasons why recognizing symptoms of dissociation in children is challenging: it may be difficult for children to describe their internal experiences; caregivers may miss signals or attempt to conceal their own abusive or neglectful behaviors; symptoms can be subtle or fleeting; disturbances of memory, mood, or concentration associated with dissociation may be misinterpreted as symptoms of other disorders.

In addition to developing diagnostic tests for children and adolescents (see above), a number of approaches have been developed to improve recognition and understanding of dissociation in children. Recent research has focused on clarifying the neurological basis of symptoms associated with dissociation by studying neurochemical, functional and structural brain abnormalities that can result from childhood trauma. Others in the field have argued that recognizing disorganized attachment (DA) in children can help alert clinicians to the possibility of dissociative disorders.

Clinicians and researchers also stress the importance of using a developmental model to understand both symptoms and the future course of DDs. In other words, symptoms of dissociation may manifest differently at different stages of child and adolescent development and individuals may be more or less susceptible to developing dissociative symptoms at different ages. Further research into the manifestation of dissociative symptoms and vulnerability throughout development is needed. Related to this developmental approach, more research is required to establish whether a young patient’s recovery will remain stable over time.

A spectrum disorder is a mental disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered "normal". Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.

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