Psychotic and bipolar affective disorders are considered severe mental illnesses with a long-term course and fluctuating presentation. Psychotic disorders are particularly characterized by significant changes to beliefs, cognition, and perception, and bipolar affective disorders are characterized by episodes of elevated mood (mania) and depression, as well as interepisode mood fluctuation. The symptoms and impacts of these disorders can be wide-ranging and complex and their presentations highly varied. Assessors need to be clear about the rationale, aims, and scope of their assessment in order to select appropriate assessment instruments. Generally, valid and meaningful assessments take a holistic approach and are built on thoughtful and sensitive engagement with interviewees. This chapter reviews pertinent issues and common assessment instruments for categorizing, quantifying, and formulating psychotic and bipolar affective disorders.