This section contains information about the definition, epidemiology, etiology, symptoms, diagnosis, classification, and prognosis of depression. Depression is a psychiatric condition characterized by depressed or anhedonic mood for at least two weeks, as well as symptoms and loss of functionality in many areas. The concepts of “melancholia” and “black bile”, which also include the symptoms of depression, were formed during the Hippocratic period and were shaped throughout the historical process. With the formation of the DSM and ICD diagnostic systems in the twentieth century, the diagnostic criteria and classification of depression took their final form. According to the current data from the World Health Organization, the 1-year prevalence of unipolar depression is 6.6%, and the lifetime prevalence of depression is 16.2%. Genetic factors such as polymorphism in MAO-A and BDNF genes, inadequate functioning of monoamines, monoamine receptor up-regulation, endocrine system irregularities such as HPA anomaly, psychosocial factors such as loss of parents in early childhood, self-perception and hopeless schemas about the future are involved in the etiology of depression