Cognitive Behavioural Therapy (CBT) has established itself as one of the most effective therapies for treating a wide range of psychological disorders. However, research and treatment in this field typically adopts a DSM driven 'disorder-focused' approach - researchers and clinicians target a specific disorder, try to understand its aetiology and maintenance, and try to develop more effective strategies to treat the disorder. This book proposes an insightful and original approach to understanding these disorders, one that focuses on what they have in common. Instead of examining in isolation, for example, obsessive compulsive disorders, insomnia, schizophrenia, it asks - what do patients with these disorders have in common? It takes each cognitive and behavioural process - attention, memory, reasoning, thought, behaviour, and examines whether it is a transdiagnostic process - i.e., serves to maintain a broad range of psychological disorders. Having shown how these disorders share several important processes, it then describes the practical implications of such an approach to diagnosis and treatment. Importantly it explores why the different psychological disorders can present so differently, despite being maintained by the same cognitive and behavioural processes. It also provides an account of the high rates of comorbidity observed among the different disorders. This book provides a novel review and integration of the empirical literature and gives clinicians and researchers a valuable new theoretical base for assessing and treating psychological disorders.