This chapter reviews the current evidence for addressing cannabis use among people with a psychotic illness. Understanding the interplay between cannabis use and mental illness, and then deciding on which interventions to employ, requires careful analysis of a number of complex factors that include current mental state, the nature and extent of cannabis use, reasons for use and readiness to change. The components of assessment vary widely from service to service and setting to setting, depending on the information sought by the clinician and/or researcher. A number of diagnostic interview schedules exist to assess the presence of both cannabis-use disorders and mental disorders in a single assessment. Assessment of readiness to change is another crucial consideration when treating substance use among people with psychotic illness. A recent Cochrane review identified just one randomized controlled trial that evaluated treatment interventions for cannabis use specifically among people with psychotic illness.