This volume in the SPIE Tutorial Text series presents a practical approach to optical testing, with emphasis on techniques, procedures, and instrumentation rather than mathematical analysis. The author provides the reader with a basic understanding of the measurements made and the tools used to make those measurements. Detailed information is given on how to measure and characterize imaging systems, perform optical bench measurements to determine first- and third-order properties of optical systems, set up and operate a Fizeau interferometer and evaluate fringe data, conduct beam diagnostics (such as wavefront sensing), and perform radiometric calibrations.