Book excerpt: Provides a complete overview of the principles, hardware, measurement methods, and clinical applications of three-dimensional dosimetry. Explains basic concepts with emphasis on 3D dose measurements and validation of 3D dose calculations as a key application of 3D dosimetry. Discusses accuracy requirements for 3D dosimetry in advanced radiotherapy as well as important topics such as audits, quality assurance, and testing. Presents state of the art detector and point detector instruments and systems, gel dosimetry, and electronic portal imaging device dosimetry. Addresses the main measurement approaches, from small-field dosimetry to 4D dosimetry, Monte Carlo techniques, and methods for quantifying differences in 3D dose distributions.