The results of some experiments on millimeter wave and light generation by means of an undulator are described. After a brief survey of the theoretical background the design of a magnet system is discussed. An experiment is described in which a 100‐Mev electron beam from the Stanford linear accelerator passed through the undulator. Light radiated by the beam was observed and the plane of polarization determined. A small linear accelerator with good bunching action was used for an experiment on millimeter wave generation. At a beam energy of 3 Mev, radiation in a wavelength band below 1.9 millimeters was observed. A peak power output of the order of one watt was obtained. Millimeter waves generated in the accelerator tube were also observed.