Persons defined as having either substance abuse, dependency or addiction, but not misuse. There were no limitations on age or other participant characteristics. The term substance refers to a drug of abuse, a medication, a toxin or alcohol, excluding nicotine. The reason for excluding nicotine, is that there is an existing Cochrane review on motivational interviewing for smoking cessation (Lai 2010). According to International classification of Diseases version 10 (ICD-10) (WHO 1993) we included the following codes, F10 to F19*, excluding F15 (caffeine) and F17 (tobacco). Equivalent disorders and codes in the Diagnostic and Statistical Manual of Mental Disorders, third revised edition (DSM-III-R) (APA 1987) and fourth edition, (DSM-IV) (APA 1994), chapter Substance-Related disorders, were also included. We also included studies in which substance abuse was not formally diagnosed. Participants could be dual diagnosis clients. We included both participants who only abuse substances and participants who also have mental problems, but we analysed the two groups separately.