The development of advanced treatment techniques including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) resulted in the implementation of more advanced treatment planning and delivery processes. In particular, complex MLC motions and high local dose modulation are characteristics of these techniques that require commissioning and routine quality assurance to safely implement into the clinic. A critical aspect is the development of patient-specific quality assurance, PSQA, which aims to check the accuracy of the IMRT plan dose calculations to detect clinically relevant errors in treatment delivery. PSQA has been a staple of the clinical workflow, and involves irradiating treatment plans onto a detector, and comparing the dose and/or dose distribution with the treatment planning system. Since the 1990s, detectors used for PSQA have evolved from point-dose measurements detectors to three-dimensional detector arrays. The development of SBRT/SRS treatment deliveries has further pushed detector fabrication to reduce the size of detector and detector spacing to improve spatial resolution. The comparison between measured and planned dose distributions requires a method, such as a gamma analysis, and tolerances that not only allow for variations in measurement but also catch potential errors. This chapter discusses the principles of PSQA, measurement techniques and detectors, analysis and tolerances, and future directions of PSQA.