High average power frequency conversion using solid-state nonlinear materials is discussed. Recent laboratory experience and new developments in design concepts show that current technology, a few tens of watts, may be extended by several orders of magnitude. For example, using KD*P, efficient doubling (> 70 percent) of Nd:YAG at average powers approaching 100 KW is possible; for doubling to the blue or UV regions the average power may approach 1 MW. Configurations using segmented apertures permit essentially unlimited scaling of average power. High average power is achieved by configuring the nonlinear material as a set of thin plates with a large ratio of surface area to volume, and cooling the exposed surfaces with a flowing gas. The design and material fabrication of such a harmonic generator is well within current technology. F INTRODUCTION REQUENCY conversion is a useful technique for extending the utility of high power lasers. It utilizes the nonlinear optical response of an optical medium in intense light fields to generate new frequencies. Frequency doubling, tripling, and quadrupling generate a single harmonic from a given fundamental high power source. The closely related processes of sum and difference frequency generate also generate a single new wavelength, but require two high power sources. These techniques have been used to generate high power radiation in all spectral regions, from the UV to the far-IR. Optical parametric oscillators and amplifiers generate two waves of lower frequency. They are capable of 'generating a range of wavelengths from a single .frequency source, in some cases spanning the entire visible, and near-infrared regions. The materials used for frequency conversion are often also suitable for other devices, such as electrooptic Manuscript