Comorbidity can be defined most generally as the co-occurrence of two or more mental health problems. It has emerged as a major clinical, public health and research issue over the past few decades. This is due in part to changes in psychiatric nomenclature, in which there is a greater focus upon elucidating any number of mental health problems with which an individual might present, rather than diagnosing one problem to the exclusion of others.

Currently, mental health problems are conceptualised as patterns of behaviour or thought that are associated with significant disability, distress, loss of individual freedom, or adverse events such as death; and which arise from dysfunction within the individual (Neugebauer, 1999). These problems can encompass a wide range of behaviours including substance use, mood disturbances, anxiety, and disturbances in thought and perception.

According to current classification systems in psychology and psychiatry, mental disorders are diagnosed according to operationalised diagnostic criteria, and the diagnosis of one disorder does not necessarily preclude the diagnosis of another (American Psychiatric Association, 1994; World Health Organisation, 1993). In some cases, more than one mental disorder is diagnosed — such comorbidity is examined in this chapter. Specifically, this chapter will define the concept of comorbidity; discuss the implications of comorbidity for theories of mental health, treatment and prevention; give a brief overview of epidemiological research into comorbidity; and examine the reasons why comorbidity might occur.