Thick phase gratings have a variety of possible uses as device components in integrated optical systems. The applications make use of one or more of the unique properties of these gratings: high diffraction efficiency, good angular selectivity, and straightforward fabrication. In planar waveguides, the diffraction efficiency and angular selectivity desired for a particular application determine the length of the grating along the waveguide surface. We have holographically recorded 2 mm gratings having peak diffraction efficiencies in excess of 50% and angular selectivities less than 2 mrad by intersecting guided waves in a sample of photorefractive LiNbO3. Such gratings provide the central element in a number of actual and contemplated waveguide devices, including mirrors, beam splitters, lenses, switches, modulators, and interferometers.