'Comorbidity' was defined by Feinstein (1970) as "any distinct clinical entity that has co-existed or that may occur during the clinical course of a patient who has the index disease under study" (pp. 456–7). Within psychiatry, comorbidity is commonly used to refer to the overlap of two or more psychiatric disorders (Boyd, Burke, Gruenberg, et al., 1984). Comorbidity between substance use disorders and other mental disorders has gained increasing prominence in psychiatry and psychology within the past few decades (Wittchen, 1996). Angold and colleagues have recently drawn a distinction between two types of comorbidity (Angold, Costello, & Erkanli, 1999). Homotypic comorbidity refers to the co-occurrence of mental disorders within a diagnostic grouping (Angold et al., 1999). The co-occurrence of two different substance use disorders (e.g. cannabis and alcohol) is an example of homotypic comorbidity. Heterotypic comorbidity refers to the co-occurrence of two disorders from different diagnostic groupings (Angold et al., 1999). This might include, for example, the co-occurrence of a substance use disorder and an anxiety disorder.