The monograph presents the main psychological characteristics accompanying depression and playing a role in its development. Biological factors are also discussed. The author cites data from epidemiological studies on depression and her own research on the prevalence of depressive symptoms in five independent samples comprising 2,159 individuals. One of the most influential theories on the psychological mechanisms underlying depression, Beck's cognitive model, is presented. Biological theories on the onset of depression are explored, and the interrelationship between negative emotional states, psychosomatic complaints, and depression is demonstrated. The main approaches to treating this "modern" disease are pharmacological, psychotherapeutic, and combined. The book will enrich the knowledge of researchers in the field of psychology, students, and doctors, especially given the seriousness of the problem and the lack of literature on it.