Debates on decisional capacity in people with mental illnesses or substance use disorders have tended to be heated. Yet, they are often based not on empirical data but on personal opinions and experiences. The empirical database in this area is quite limited, but has been growing in recent years. The following discussion focuses on relevant clinical investigations. We consider variations across and within different diagnoses, barriers to decision-making, methods for assessing capacity-interview versus instruments, choosing from among different capacity instruments, decisional capacity-is it a state or a trait?, triggers for assessment of decisional capacity, methods for enhancing capacity, and decisional capacity in people with substance use disorders. Finally, we discuss some relevant health policy recommendations.