This Special Issue book includes two reviews and six research articles. The review by Götze et al. (2020) provides a comprehensive compilation of the mineralogical and geochemical data of agates, occuring worldwide and in a range of different host rocks, and summarizes the results of extensive studies on agates of different origin and type. Moxon and Palyanova (2020) present a thorough analysis of the genesis of agates in basic volcanic rocks, including relevant literature covering the last 250 years. The research articles considered in this paper contain detailed studies of agates from regions as diverse as Morocco, China, Russia, and Cuba. These investigations add to the knowledge about the genesis of agates. The Guest Editor hopes that the articles in this Special Issue will be a helpful and valuable resource for anyone who is interested in the genesis of agates and will provide a foundation for further research. The Guest Editor would like to thank the authors of all of the articles, the Editor-in-Chief, the Editor-in-Chief of the Mineral Deposits section, the Editorial Board, and all the reviewers for their work on this Special Issue. In addition, the Guest Editor is especially grateful to the late Terry Moxon for his invaluable help.