The article is devoted to the study of the phenomenon of medism. The terms «medismos» and «medizein» were used by the Greek authors to designate collaboration with the Persians. The greatest part of references to medism occur in Herodotus' History, which may be consid-ered as the main source for our understanding of the phenomenon. Thucydides mentions me-dism only four times, other Greek writers mention it occasionally when treating the Persian wars. An analysis of the contexts in which Greek authors used the term «medism» leads the author to the following conclusions. One may make a distinction between the medism of Greek states or ethnic groups and indi-vidual medism of some prominent persons. Herodotus, for the greatest part, mentions state medism (of Aegina, Thessaly, Thebes and other Boeotians) and gives full explanation only to these cases. Medism had been caused by three main reasons: a) great fear of Persian invasion; 34 b) the «medization» of Greek political elite; c) traditional rivalry of various Greek states and ethnic groups, which prevented them from uniting against the Persian enemies. Herodotus rarely comments on individual medism (the Aleuadai, Attagines and Timegenides etc.) and never mentions its reasons. Medism and treason (prodosia) are not directly interrelated. The terminology of medism was applied to medizing states and peoples who sided with the Persians under the pressure of military threat, but only those of them which collaborated with the enemy deliberately may be considered as traitors. The author of the article considers as well the possibility of applying the definition of me- dism to the Greek attitude towards Persian dress and customs. It may be concluded that the Greek perception of Persian mores could only put somebody under suspicion of medism, but it was not sufficient for an official accusation of medism as of a crime (treason). In general, the phenomenon of medism reflects the development of Greco-Persian antago-nism; it came to its end after Callias' peace treaty of 449 ВС, and the Spartan-Persian treaties of the Peloponnesian War opened the era of diplomatic involvement of Persia in the Greek affairs.