The issues of nosology, systematics, and clinical and social significance of dysthymia are a serious problem of modern medicine. Dysthymia significantly contributes to the structure of depressive disorders. Although its symptoms are mild, the disease is associated with high risks of disability, concomitant health problems, and mortality. There is no established system of classifying and diagnosing chronic mild depression. This review presents epidemiological data, historical review, and nosology, along with the assessment of persistent depressive disorder. Full-text articles and fragments of monographs selected by keywords in the databases Scopus, Web of Science, MEDLINE, RSCI, Psychiatrist, ScienceDirect were used for research.