This article is a survey of the characteristics, technology and applications of Holographic Optical Elements (HOEs), considered as general elements in an optical system. HOEs function by diffraction of light from a generalized grating structure with nonuniform groove spacing. HOEs provide a system of thin film optics. They are capable of unique system functions and configurations, show a rapid variation of optical power and image characteristics with wavelength, have relatively large amounts of astigmatism and coma, and require special consideration of optical efficiency during system design. Comparison of the aberrations of specific, f/3.3 elements shows that the on-axis HOE and the conventional glass lens element have similar aberration levels, while the off-axis HOE has four times as much astigmatism and twice as much coma. We show that these grating aberrations, which appear for conjugate points that are different from the HOE construction points, are proportional to the average surface grating spatial frequency of an off-axis HOE. HOE technology is similar to conventional optics technology, but is less developed. The relative complexity of optical systems with HOEs, and the lack of a suitable aberration theory, produce a relince on computer-based raytracing for system design and development. We give the basic raytracing equations and discuss the special requirements for hologram recording appara-tus and materials. In applications, HOEs will usually provide unique capabilities rather than replace conventional elements, and will usually operate over narrow spectral bandwidths. The use of HOEs in several types of laser optical systems, and in visual displays, appears to be not only advantageous, but also technically and economically feasible.