The present paper examines the diachronic development of Serbian clitics. The investigation of clitics is of special interest in Slavic languages: despite the fact that these languages display free worder, the use of clitics is subject to strict rules. Clitics were present already in the Proto-Slavic language, however, in the course of the history of the language they were totally lost in Eastern Slavic and also partially in Western Slavic. The array and function of Serbian clitics has undergone substantial change: (1) similarly to Proto-Slavic, in Old Serbian manuscripts three types of enclitics were used, namely, discourse, pronominal and auxiliary clitics; and (2) clitics in present-day Serbian (PDS) tend to cluster and, within the clusters, their order is regulated by strict rules. This ordered position of clitics within the clusters has undergone changes. Moreover, there are significantly fewer clitic clusters in the investigated manuscripts than in PDS texts. The investigation of clitics is carried out in a 15th century Serbian manuscript, the Troyan Parable (Троянска притча) and in Old Serbian documents from the Middle Ages.