Development of plastic scintillation detector (PSD) systems for dosimetry has been evolving for two decades now. The subject area has been a topic of a keen research interest that has generated lot of publications lately. PSD systems are being introduced in the market place. Numerous PSD systems for External Beam Radiation Therapy and Brachytherapy have been proposed, most times differentiating from the original systems by slight changes in one or more components, such as the photodetector. However, few major technological and engineering innovations have enabled this new technology to be embraced by the community which helped launching these systems for commercialization (Standard Imaging, Radiadyne, etc.). Scintillation materials (plastic, scintillating fibers and liquid) have many properties that make them ideal for dosimetry including water equivalence and energy independence for MV photons, linearity with dose, dose rate independence, high spatial resolution and nanosecond fast response time. Therefore, these detectors do not require the usual conversion and/or correction factors used for other commonly used detectors to convert the dosimeter reading to absorbed dose. This evolution started with point detectors and has led to matrix arrays to respond to the ever-increasing complexity of radiotherapy treatment fields such as IMRT, small field dosimetry or incorporating them within special patient applicators to monitor the dose in real time. This lecture will provide a brief overview of the dosimetric characteristics and properties of PSDs when exposed to high-energy photon beams. We will present few recent applications of PSD systems as applied for EBRT and Brachytherapy (quality assurance and in vivo dosimetry applications). We will briefly present the available and soon to come commercial products related to this type of dosimetry. LEARNING OBJECTIVES 1. Review the underlying physics of organic scintillation materials and the properties of plastic scintillation detectors used in radiation dosimetry. 2. Understand the principles of the methods associated with recent innovations of PSDs in EBRT and Brachytherapy. 3. Identify potential applications that could benefit from these PSD systems.