This book is a practical introductory guide for graduate students and novice psychotherapists in clinical and counseling psychology. It provides guidance on how to diagnose mental disorders using the DSM-5-TR and addresses current issues and controversies in diagnosis. The book also contains detailed guides on how to treat clients with common mental disorders, including depression, anxiety disorders, posttraumatic stress disorder, obsessive-compulsive disorder, insomnia, hoarding disorder, and others. The book emphasizes the importance of evidence-based diagnosis and treatment in the context of a warm and supportive therapeutic relationship.