Detailed microscopic studies of rocks from virtually all terrestrial environments show that one or more minerals in these rocks contain small quantities of liquid (or glass) and/or vapor trapped in cavities generally less than 50 microns in diameter. These are fluid inclusions, and they provide us with samples of the fluid phase present in the rock at some time during its evolution. Early petrographers (Sorby, 1858; Rosenbusch, 1923; Zirkel, 1873) included descriptions of these features. More recently, several individuals (notably Lemmlein, Dolgov, and Ermakov in the U.S.S.R.; Poty, Touret, Touray, and Weisbrod in France; and Roedder in the United States) have emphasized the petrologic information that can be obtained from these fluid inclusions. Two recent publications synthesize and summarize much of the previous work on fluid inclusions: the Mineralogical Association of Canada short course handbook Fluid Inclusions: Applications to Petrology (edited by Hollister and Crawford, 1981) and the Mineralogical Society of America Review in Mineralogy entitled Fluid Inclusions (Roedder, 1984). In this chapter, we emphasize important contributions toward understanding metamorphic processes that have been established through study of fluid inclusions, and we review several areas of ongoing research in which the study of fluid inclusions contributes to discussions of unresolved problems.