The subject of this volume is the religion of the Celts as it is depicted in ancient literary sources. In addition to epigraphic monuments and archaeological findings, our knowledge of this part of Celtic life is mainly based on Greek and Latin literary testimonies. This material was collected for the first time in the 1930s by Johannes Zwicker in his Fontes Historiae Religionis Celticae. However, this collection, which has not yet been reprinted, lacks many relevant passages; moreover, it is merely a text collection and contains no commentary. The aim of the present study is to fill these gaps. The Greek and Latin testimonies have been translated into German to aid in their understanding. In addition, the chronologically arranged passages are furnished with a detailed commentary, in which these often problematic testimonies are analyzed in detail based on recent research. Questions of textual criticism and verification of sources are also taken into consideration. The individual testimonies are analyzed within the larger contextual framework, and internal text references and special aspects of the individual literary genera are taken into account. The textual analyses interpret the information found in each testimony.