This chapter considers the principles of interferometric dimensional metrology applied to surface features best viewed in a microscope. Interference microscopy encompasses a wide variety of techniques for measuring surface texture, shape, step heights, lateral positions and dimensions, transparent film structure, and the scattering behavior of optically unresolved patterns. The physical concepts and fundamental instrument geometries underlying interference microscopy have a history reaching back to the nineteenth century, but the more recent developments of the technique link closely to rapidly advancing enabling technologies, particularly in digital data processing. Increased precision and greater surface structure complexity require something more automated than visual inspection of lines of equal interference intensity. Electronic data acquisition together with computerized data analysis allows for quantification of surface texture at the nanometer level, over surfaces that may have highly variable reflectivity and structure.