The purpose of this article is to review some of the novel flow visualization techniques that are particularly suited for studying unsteady flows. While many of these techniques are equally conducive for steady flow fields, the present paper will emphasize applications to time-dependent flows. Possible pitfalls in interpreting the visualization results are elaborated. The flow visualization methods presented include those that make use of scattered light from small particles or other foreign additives as well as those that make use of changes in the index of refraction.