Research Design and Methods The proposed 5-year study is a randomized, controlled trial of screening, brief intervention, and referral to treatment (SBIRT) for alcohol, cannabis and stimulant use in mental health treatment settings. These 3 substances have been chosen as the focus for the proposed study because (a) in the US alcohol and cannabis are the most common substances used by individuals with serious mental illness in the general population1 and (b) stimulant use (in particular, methamphetamine) is a significant public health threat in the US. 31,32 The SBIRT model will be tested in a two-group design in which eligible participants (N=750) will be randomly assigned to either the SBIRT intervention condition (N=375) or to a control condition (N=375) that offers an informational leaflet. The participants will comprise male and female mental health patients who have a positive pre-screen for being at risk of having or developing a substance use disorder. The performance site for the study will be publicly-funded mental health clinics in Ventura County, California. All participants (intervention and control groups) will receive mental health care as usual and their participation in the study will in no way reduce the level of usual care they receive.