The term dual diagnosis is used to describe the comorbid condition of a person considered to be suffering from a mental illness and a substance abuse problem. There is considerable debate surrounding the appropriateness of the term being used to describe a heterogeneous group of individuals with complex needs and a varied range of problems. The concept can be used broadly, for example depression and alcoholism, or it can be restricted to specify severe mental illness (e.g. psychosis, schizophrenia) and substance misuse disorder (e.g. cannabis abuse).

Dual diagnosis is also used to describe a co-occuring condition in which a person is simultaneously diagnosed with an Axis I and an Axis II psychiatric disorder. While Axis I conditions are considered more or less amenable to treatments such as individual therapy and psychotropic drugs (e.g., antipsychotic, anxiolytic, and antidepressant medications), Axis II conditions are typically considered more resistant or even refractory to such treatments. Common Axis I conditions that may be treated though drug therapy, counseling, or a combination of the two include (but are not limited to) Major Depressive Disorder, Obsessive-Compulsive Disorder, Generalized Anxiety Disorder, Delusional Disorder, and Schizophrenia. Axis II conditions are limited to Mental Retardation and the personality disorders such as Borderline Personality Disorder and Antisocial Personality Disorder. These conditions were originally separated from the Axis I conditions to highlight their intractability to treatment, although there is some evidence to suggest that personality disorders may be managed through long-term individual therapy. The fact that Autistic Disorder is coded on Axis I is one of the many criticisms of the DSM-IV-TR (the diagnostic manual for mental disorders published by the American Psychiatric Association), as this falsely implies that Austic Disorder can be "cured" through popular but fad treatments.