Depression is a condition that results in low mood and aversion to activity. It can be long term and short term. Depression can cause sadness, difficulty in thinking and concentration, a feeling of dejection and hopelessness. It also affects a person's behavior, thoughts, motivation, feelings, and sense of well-being. Some factors that cause depression are life events, gender identity and sexuality, medical treatment and substance-abused. The three major types of depression are bipolar disorder, major depressive disorder and persistent depressive disorder. Bipolar disorder is the condition in which a person experiences irregular states of depression and mania. The major depressive disorder is a condition with severe symptoms that disturb the daily life and affects a person's appetite, ability to enjoy life, sleep, work, etc. Depression can be treated with the help of psychotherapy and psychotropic medication, especially antidepressants. This book provides significant information of this condition to help develop a good understanding of depression and related disorders. It presents researches and studies performed by experts across the globe. This book is a vital tool for all researching and studying psychiatry.