It can be argued that an individual’s subjective experience and beliefs about a substance are important. Motives and expectancies regarding the use of alcohol and drugs are often that they are going to modify a cognitive state or help them cope with a particular situation. However, there are growing concerns in the U.S.A. and in the U.K. regarding individuals who experience psychosis and concurrently use substances. Correctly diagnosing individuals with dual presentation is said to be difficult, engagement in treatment is problematic, and medication adherence and prognosis poor. In this paper a cognitive-developmental model is proposed. I suggest that for individuals who experience psychosis and also use drugs or alcohol, the ability to identify the relationship between the substance use and the psychotic illness in terms of a case formulation/conceptualization would provide a good starting point for developing strategies and interventions that are most likely to succeed in treatment. Such an approach would explicitly address key cognitions. Unless the dysfunctional substance-related beliefs are addressed, adherence to medication and engagement with treatment services will be hindered and the possibility of relapsing to problematic substance use and acute psychosis remains. A cognitive treatment component, to target these beliefs, based on the cognitive model of substance misuse and the motivational interviewing approach will also be briefly outlined.