Microelectromechanical Systems ("MEMS") and optics are a natural match. There are several reasons: MEMS devices have dimensions and achievable actuation distances comparable to the wavelength of light; smooth-surfaced dielectrics, semiconductors, and metals can be used in various combinations; and, photons don't weigh anything, so relatively feeble MEMS actuators can easily manipulate them. Many optical MEMS devices are based on mirror arrays that can be tilted using electrostatic actuation. This paper, however, focuses on programmable diffraction gratings and their uses for projection displays, spectroscopy, and wavelength management in modern optical telecommunication systems.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.