This chapter aims to give a general and comparative overview of the most prominent cases of philosophical influence on medicine during the Roman period, mediated principally by the so-called medical haireseis, or “sects.” It will focus on the most influential medical sects and authorities that either grew up in Rome or that seem to have developed significantly there, looking at each in turn—namely the Asclepiadeans, Pneumatists, Empiricists, Methodists, and Galen. The individuality of each will be emphasized, through both their doctrines or methodologies and their later impact, and the ways in which they reacted to each other and interacted with the philosophical schools.