Obsessions, compulsions and related phenomena occur across a wide spectrum of neuropsychiatric disorders. The boundaries between obsessive-compulsive disorder (OCD) and other psychopathological phenomena, such as delusions, impulsions and habits, remain unclear. Further, the subclinical symptoms of OCD are highly prevalent, causing significant impact but yet are poorly understood. To help address these limitations, recent debates have highlighted the importance of a transdiagnostic approach to psychiatry. This book integrates what is currently known about obsessionality, compulsivity and the boundaries of OCD and related disorders and unveils areas that are worthy of future research. Using a transdiagnostic framework, it provides a comprehensive review of the key issues to understanding the diagnosis and evaluation of OCD and related disorders, as well as describing how the clinician can treat OCD and its manifold presentations. Edited by leading specialists in the field, this book offers a global perspective to the diagnosis and treatment of these disorders.