Liquid-crystal optical waveguides may be modulated or switched with high sensitivity by electrooptic magnetooptic or acousto optic effects. Such guides, however have relatively slow response times and. relatively high scattering losses so that they currently appear limited to special-purpose applications. A review is given on a variety of liquid-crystal optical switches or modulators. Specific work is then described on the analysis and experiment on thin-film nematic liquid-crystal guides with propagation constant controlled by means of an applied voltage across the films. A dynamic scattering switch is also demonstrated. Recent results show switchable grating directions in films having mixtures of nematic and cholesteric liquid crystals. The controlling variable is the frequency of the applied electric field.