This book provides the first English translation of the Greek text of the Spherics of Theodosios (2nd-1st century BCE), a canonical mathematical and astronomical text used from as early as the 2nd century CE until the early modern period. Accompanied by an introduction to the life and works of Theodosios and a contextualization of his Spherics among other works of Greek mathematics and astronomy, the translation is followed by a detailed commentary, and an accessible English paraphrase accompanied with mathematically generated diagrams. The volume has a broad appeal to both general and specialist readers who do not read ancient Greek - allowing readers to understand the mathematical and astronomical principles and methods used by ancient and medieval readers of this important text. The paraphrase with its mathematical diagrams will be useful for readers with a scientific and mathematical background. This study of one of the canonical mathematical and astronomical texts of the ancient Greco-Roman, classical Islamic, and medieval Christian worlds provides an invaluable resource for historians of science, astronomy, and mathematics, and scholars of the ancient and medieval periods.