Photosynthetically Active Radiation or PAR is the part of the electromagnetic spectrum that drives photosynthesis in plants. Measuring PAR is an important task in farming, plant biology and environmental science. This article reviews the theoretical basis and various practical approaches for measuring PAR. This review also describes the principles of operation and the technology behind different types of PAR sensors. Comparisons between commercially-available PAR sensors and sensors that are in the prototyping stage are provided to give the reader a quick overview of the technologies available for PAR measurement. The aim of this review is threefold: 1) to serve as an introduction to the topic of PAR sensing for researchers new to the field, 2) to provide list of options for readers seeking to use commercially-available PAR sensors and 3) to give the theoretical foundation and technical information to those readers that seek to develop custom PAR sensing solutions.