This section describes the quality requirements for optical systems, encompassing specifications for aberrations, surface quality, material parameters, and roughness. It also introduces the most common methods for testing optical quality, such as stylus profilometry, interferometry, Hartmann-Shack sensing, and transport of intensity methods. Basic quality requirements for surface figure, roughness, alignment, and other parameters in optical systems comprising both mirrors and lenses are defined. Efficient metrology procedures to verify the implementation of these specifications are described.