Motivational interviewing has been employed extensively and successfully among people with alcohol and other drug problems in order to enhance involvement in treatment and to reduce substance use. Despite clear commonalities in motivating people with mental health problems to recognise their problem and adhere to treatment, motivational interviewing has not been widely adopted by mental health professionals, largely due to the separation of mental health versus substance use services. The existing evidence for the effectiveness of motivational interviewing in improving engagement in treatment for mental health problems is reviewed. It is recommended that, based on the evidence available, motivational interviewing should be applied to people with mental health problems as well as those with substance-related problems to enhance treatment engagement, retention and response. Guidelines for its application are suggested.