The importance of mid-ocean ridge hydrothermal systems has been recognized for their role in the regulation of ocean and sediment chemistry, as well as for providing a chemosynthetic source of carbon which drives a unique population of animals found at hydrothermal vents. Despite the importance of these systems the rates, length, and depth scale of submarine hydrothermal processes are not precisely known because they are, for the most part, inaccessible to observational tools. We must therefore rely on indirect methods to quantify these processes. One way of investigating the rates, or timescales, of processes in a hydrothermal (or any natural) system is through the study and modeling of naturally occurring radioisotopes. Disequilibria among the naturally occurring radioactive decay series in vent fluids, associated mineral deposits, and overlying effluent plume have provided geochemical tools to investigate the rates of various processes occurring in submarine hydrothermal systems. Because the half-lives of the radioisotopes vary from days to many years, processes which encompass a wide range of spatial and temporal scales can be studied. This paper presents a review of methods that estimate the residence time of hydrothermal fluids in the ocean crust, establish the geochronology of seafloor sulfide deposits, investigate the rates of chemical reactions within hydrothermal effluent plumes, and derive the heat and mass flux from seafloor hydrothermal areas. ¿ American Geophysical Union 1996