This chapter presents the properties of Polymer Optical Fibers (POFs) that are critical to their sensing performance, including their optical transmission, strain and temperature sensitivity, and their response to humidity. The majority of optical fiber sensors are based on silica optical fibers due to their wide use in telecommunication applications and the wide availability of components, instrumentation, and optical fiber specifications. POFs have experienced a surge in applications for short haul telecommunication systems. The chapter highlights the unique benefits of POF-based sensors, as well as challenges to their application. The unique benefits of POFs include the low stiffness and high ductility of polymer materials, as well as their low weight. These permit POF sensors to be applied in applications where high levels of strain are expected and applications where precision measurements must be made at very low force levels. All measurements were made in the low-deformation regime.