The combination of the techniques of holography with those of flow visualization has resulted in one of the most wide-spread uses of holography. The most significant recent contributions in the field comprise solution of practical engineering problems, refinement in analytical and interpretive methods, and an increasing list of current and potential applications. The purpose of this paper is to summarize the meth-1 ods and state-of-the-art of holographic flow visualization. Holographic interferometry, like conventional interferometry, provides a measurement of optical path length between points of space. This can be applied to determine gas density or to observe density gradients and their movement throughout a volume. Moreover, a holographic image of a space is not just limited to interferometric analysis. Such images are amenable to a broad class of optical analysis, including three-dimensional photography and optical filtering methods such as schlieren photography.