Cannabis use is highly prevalent among individuals presenting with their first episode psychosis (FEP), and a substantial proportion of individuals continue to use cannabis following the onset of the illness. Longitudinal research following up FEP patients has now started to explore in more detail the impact of cannabis use on long-term prognosis of psychosis, implicating continued cannabis use as a risk factor for an unfavourable illness course of psychosis. Since cannabis use is one factor amenable to treatment, it constitutes an attractive interventional target for early intervention. As such, a nuanced understanding of how exposure to cannabis affects the long-term course of psychosis is paramount, to help clinicians and carers to formulate the best possible treatment choices for individuals with a first episode psychosis. This chapter will focus in more detail on questions concerning the impact of cannabis use on the illness course of psychosis, including an overview of its effects on clinical and functional outcome dimensions, an assessment of causality, and a discussion on the interplay between cannabis use and other factors implicated in long-term prognosis of psychosis.