This paper presents an historical overview of the study of the Byzantine prayer book—the Euchologion. The modern study of the Euchologion began with Jacques Goar's (1601–1653) edition Euchologion, sive Rituale Graecorum... in 1647 and continued with Alexei Dmitrievskii's (1856–1929) Opisanie liturgicheskikh rukopisei..., vol. 2: Euchologia, published in 1901. Since then, various taxonomies of the Euchologion have been proposed by liturgiologists based on the liturgical environment where the book was used [Miguel Arranz (1930–2008)], the textual recensions of its liturgies and prayers [André Jacob (b. 1933)], and the regional provenance of Euchologion manuscripts [Stefano Parenti (b. 1959)], to name a few. These taxonomies are presented and analyzed in order to understand the current state of research, propose classifications of Euchologia prayers and manuscripts, and indicate questions and desiderata for further research.