Psychiatric diagnosis is experiencing a crisis of confidence. Current approaches are outmoded, with reform desperately needed. Clinical staging is a solution to this crisis. Clinical staging addresses the limitations of current diagnostic systems by recognising the full continuum or trajectory of mental illness from asymptomatic to chronic illness. It acknowledges the overlap between mental health symptoms during early stages and directly links each stage to treatment and underlying cognitive, neurological and biological changes. This approach enhances chances of early identification, promotes the implementation of safer treatments, and increases opportunities to alter the negative trajectory of mental disorders. This book comprehensively describes the conceptual basis of clinical staging in psychiatry, details current progress in identifying biomarkers for each stage, and explores the implications of staging on treatment and health systems. This book provides a foundation for transformational reform in psychiatric diagnosis.