Delusional beliefs are key symptoms of mental illness, and physicians over hundreds of years have attempted to understand and offer treatments for patients with such beliefs. In this chapter, the authors will explore the experience of delusional beliefs within the clinical context: i.e. with people who present to mental health services for help. The authors begin with definition of the descriptive psychopathology, prevalence and context in which delusional beliefs occur and their clinical relevance. Delusional beliefs have a core role in distress, depression and risk within psychosis. Real examples will be used to reflect on both the form delusional beliefs take and their personal content, concluding with treatment options and challenges.