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Mental illnesses are classified according to the symptoms that a patient experiences, as well as the clinical features of the illness. All recognized mental illnesses have been assigned diagnostic criteria by the American Psychiatric Association (APA). Major categories of mental illness include:

• Anxiety disorders. Conditions in which patients feel anxious or distressed for no logical reason. People with these disorders may experience chronic, intense and irrational anxiety on a regular or even daily basis. As a result, anxiety disorders can seriously diminish quality of life, hampering a person’s ability to work, travel and form and maintain interpersonal relationships. Examples include post-traumatic stress disorder (PTSD), panic disorder, obsessive-compulsive disorder (OCD), generalized anxiety disorder (GAD) and various specific phobias.
  
  
• Cognitive disorders. Conditions marked by impaired awareness, perception, reasoning, memory and judgment. Patients with these disorders have difficulty with the basic processes of thinking. Examples include delirium, dementia and amnestic (amnesia) disorder.
  
  
• Developmental disorders. Conditions that delay or impede a child's physical or psychological development. They begin in childhood and involve a wide range of symptoms. Examples include  autism, attention-deficit hyperactivity disorder (ADHD) and learning disabilities.
  
  

  

• Dissociative disorders. Conditions in which patients frequently escape reality by suppressing their memories or taking on other identities. This dissociation occurs when a group of normal mental processes becomes separated from other mental processes. Examples include dissociative amnesia, dissociative fugue, dissociative identity disorder ([DID] formerly known as multiple personality disorder) and depersonalization disorder.
  
  
• Eating disorders. Conditions that involve serious disturbances in eating behavior, including unhealthy reduction of food intake, severe overeating and/or dangerous methods to prevent weight gain, such as self-induced vomiting. Examples include anorexia nervosa, bulimia nervosa, binge eating disorder, rumination disorder and pica.
  
  
• Mood disorders. Conditions with an abnormal mood as their primary feature. “Mood” refers to the sustained feelings and emotions through which a person interprets life. Most mood disorders are defined and diagnosed by the occurrence of one or more mood episodes, or periods of abnormal happiness or sadness. Examples include major depression, bipolar disorder, dysthymia and seasonal affective disorder.
  
  

  

• Personality disorders. Conditions in which a person struggles to deal with other people and acts in a manner that is inflexible and not well-suited to coping with the demands and changes of life. Personality disorders begin by early adulthood, remain stable over time and create distress or impairment in a person’s life. Examples include borderline personality disorder (BPD), antisocial personality disorder and avoidant personality disorder (APD).
  
  
• Schizophrenia. Chronic mental disorder that makes it difficult to distinguish between real and imagined experiences, think logically or have normal emotional responses to situations.
  
  
• Sexual disorders. Conditions that prevent people from having rich and fulfilling sexual relationships. These disorders involve problems related to sexual functioning, desire or performance. Examples include sexual dysfunctions, paraphilias and gender identity disorders.
  
  
• Sleep disorders. Conditions that involve include difficulty falling or staying asleep, falling asleep at inappropriate times, sleeping excessively or abnormal behaviors during sleep. Examples include dyssomnias (e.g., insomnia, narcolepsy), parasomnias (e.g., night terrors, sleepwalking), sleep disorders associated with medical or psychiatric conditions and proposed sleep disorders (sleep problems for which there is insufficient information to establish them as distinct disorders).
  
  
• Somatoform disorders. Conditions in which individuals experience symptoms that cannot be attributed to a physical cause. Hypochondriasis is an example of a somatoform disorder.
  
  
• Substance abuse disorders. Problems that are associated with the abuse of alcohol, nicotine, caffeine, nonprescription medications, prescription medications and illegal drugs.