This book offers a comprehensive, evidence-based approach to understanding depressive disorder, integrating research, clinical guidelines, and practical treatment strategies. It is structured into four sections, covering fundamental concepts, diagnostic challenges, special populations, and treatment approaches. The book aims to support professionals involved in managing depressive disorders by providing a broad and in-depth understanding of the condition. The Part 1 explores the concept and diagnosis of depressive disorder, including distinctions between bipolar and unipolar depression, the concept of treatment-resistant depression, and mixed specifier. The Part 2 examines how depressive disorder manifests across different demographics, including women, the elderly, children, and individuals with comorbid conditions such as physical illnesses. The Part 3 is dedicated to pharmacological treatment strategies, detailing novel antidepressants, psychedelics, and microbiome. The last Part addresses broader issues such as the risk of suicide, the effects of parental depression on child development, and trauma.