This analysis examines several sources of random and systematic errors present in most interferometer systems. Optical cavity errors are typically the primary limitation on measurement accuracy. Secondary sources of error include imaging distortion, ray-mapping errors, and detector noise. When necessary, differences between phase and fringe measuring systems are addressed. The analysis is kept as generic as possible for both fringe and phase measurement, though quantitative analysis of some error sources is instrument dependent. Such analysis is directed toward the Zygo MARK interferometers and Zygo ZAPP processor. The use of reference subtraction, distortion calibration, and averaging for improving both accuracy and precision are also discussed. Ray-mapping analysis indicates practical limits on the amount of tilt, power/focus shift, and asphericity in the test piece or aberrations in the wavefront of the light source, which may be tolerated without significantly degrading measurement accuracy. The role of absolute testing techniques is also discussed.