Lupton [this volume] reviews the nature of hydrothermal plumes formed above a mid-ocean ridge. Among the reasons for studying plumes is that their existence provides a means of locating new vent systems. Moreover, plumes integrate the thermal and chemical output from a vent field, that is, they contain a signal averaging the contributions from the various styles of discharge, for example black smoker jets, percolation of fluid through sulfide structures, and diffuse flow from fissures in the basaltic substrate. Thus, their study offers the potential to evaluate fluxes of heat and mass from vent systems in an efficient and cost effective way. But to realize that potential requires an understanding of the physical processes involved in developing a plume. In this paper I consider the physical processes involved in plume development and how these relate to observations that can help to answer questions about the underlying volcanic-tectonic-hydrothermal system.