Much of the research that informs current understandings of drug and alcohol use and addictive behaviour is based on studies that concentrate on a single substance type. This narrow focus is in distinct contrast to actual patterns of use and related harms: most Canadians with substance use problems use more than one substance (polysubstance use), often on the same occasion, and their behaviours and health outcomes may be strongly shaped by this combination. Health policies and programs are usually developed, implemented and evaluated one substance or behaviour at a time, without consideration of possible consequences for other substance use and addiction outcomes. In the current research literature there is a lack of information regarding polysubstance use. This includes when, in what order, and in what quantities people use substances, why they choose to use simultaneously, and the risk behaviours (e.g. sexual behaviour, spending behaviours) associated with simultaneous polysubstance use. Kristina Brache is exploring the patterns of use, the settings and the motivations associated with combined use of alcohol and psychostimulants (cocaine, amphetamines). She is conducting a series of in-depth interviews and self-administered surveys of 150-200 substance-using individuals in a treatment setting. Understanding the use of multiple substances can inform prevention and intervention strategies to reduce harm or risk to individuals using multiple substances. Ultimately, this research could improve health and health delivery systems by informing policy, programs and treatment about combined drug use in this population of interest.