Less than 1% of the sea floor where hydrothermal systems are likely to be found has been systematically investigated; thus, our knowledge of the distribution of hydrothermal activity is very limited and is biased towards sites of active venting of high temperature, focused hydrothermal fluids. Submarine hydrothermal systems have now been found along fast, intermediate and slow-spreading mid-ocean ridges, at intraplate volcanic centers, and in island arc settings, both in back-arc basins and in fore-arc areas; a summary of their locations has been published by Rona and Scott [1993]. However, our ability to develop predictive models of both the distribution and temporal variability of hydrothermal systems has been limited by the lack of high-resolution datasets that can be used to compare salient features of hydrothermal fields in different tectonic settings. This has been further complicated by evidence from time-series sampling of hydrothermal fluids, observations of venting in the immediate aftermath of a volcanic eruption, and isotope dating of hydrothermal deposits, that the characteristics of hydrothermal systems, ranging from individual vents to entire hydrothermal fields, can change over time-scales of days to tens of thousands of years. Despite these limitations, some distinctive relationships are beginning to emerge from several recent studies along fast, intermediate, and slow spreading ridges.