This study comprises a translation and commentary of Book I of the Planetary Hypotheseses by the second century A.D. Greco-Roman astronomer, Claudius Ptolemy. It closely examines the Planetary Hypotheses on its own and in relation to Ptolemy's other writings. Where necessary I rely on astronomical, philosophical, and technological works by other writers in order to better situate Ptolemy's ideas into the context of Greco-Roman science.The dissertation is organized into three sections. Section I consists of an extended introduction to the Planetary Hypotheses. I offer a synopsis of the Planetary Hypotheses and a history of the text in Sections I.1 and I.2. Section I.3 consists of a brief introduction to notation and sexagesimal numbers while Section I.4 analyzes the aim and function of Ptolemy's planetary models.Section II is a translation of the existing Greek text of the Planetary Hypotheses, namely Book I Part A, and a precis of Book I, Part B. The translation is made from J.L. Heiberg's edited Greek text and the precis relies on the English translation by Bernard Goldstein, the French translation by Regis Morelon, and the Arabic Manuscripts found in the British Library (Arabic-A) and the Library at the University of Leiden (Arabic-B). The footnotes include variant readings from the different Greek and Arabic Manuscripts. A list of all existing manuscripts of the Planetary Hypotheses can be found in Section I.2While I focus on the changes that Ptolemy made to the models in the Planetary Hypotheses from his theories in the Canobic Inscription, Handy Tables, and the Almagest, this work aims to explore the motivations behind these changes. Additionally, I contextualize the Planetary Hypotheses within Greco-Roman and Islamic astronomy and technology. What emerges from this dissertation is a consideration of Ptolemy's ideas about the practice of science and an analysis of how he modeled astronomical observations.Section III is a commentary of the entirety of Book I (Parts A and B). This section is arranged so that it loosely follows the order of topics found in the Planetary Hypotheses Section III.1 examines the Planetary Hypotheses in terms of instrument-making. Section III.2 discusses the geometric models that Ptolemy presents along with a discussion of the changes that he makes. I give an overview of the period relations and mean motions presented in the Planetary Hypotheses in Section III.3 and III.4 and the new frame of reference in Section III.5. Section III.6 briefly examines Book II of the Planetary Hypotheses and Section III.7 addresses the relationship of Book I and Book II and contextualizes this work in the history of Greco-Roman science. Finally, Section III.8 examines the role the Planetary Hypotheses played in developments within Medieval Islamic astronomy.