Abstract Despite recent commercial interest in possible mining of sea-floor massive sulfide (SMS) deposits, there has been a great reluctance to attempt any estimate of their global abundance, owing to the limited exploration of the oceans and the general lack of knowledge of the deposits. The need for such an assessment is now more urgent, as a number of leading companies and international consortia have begun to invest in intensive exploration campaigns for SMS, and governments and other organizations have begun to establish the legal framework for sea-floor exploration and exploitation of mineral resources in territorial and international waters. A growing database of global SMS occurrences is beginning to provide clues to the likely distribution, size, and grade of the deposits. More than 300 sites of sea-floor hydrothermal activity and associated mineralization are now known on the ocean floor; about 200 of these are sites of confirmed high-temperature venting (black smokers) and associated polymetallic sulfide deposits. They occur primarily at mid-ocean ridges (65%) but also in back-arc basins (22%) and on submarine volcanic arcs (12%). More than 3,800 samples have been collected from 95 of the best studied deposits, and preliminary estimates of the sizes of the deposits have been made at 62 sites. The total amount of massive sulfide contained in the known deposits is estimated to be ˜50 million tons (Mt); the top 10 percent of deposits (≥2 Mt) contain about 35 Mt of massive sulfide or about 70 percent of the total. The largest deposits, excluding the Atlantis II Deep in the Red Sea, are on the order of 10 Mt in size. However, the median deposit size is only about 70,000 t. The average concentrations of metals based on analyses of surface samples are 3.6 wt percent Cu, 7.9 wt percent Zn, 0.4 wt percent Pb, 1.7 g/t Au, and 115 g/t Ag, although comparisons with drill core indicate that grades can be significantly lower below the sea floor in many, but not all, deposits. A number of independent datasets, including global heat flow, circulation models for high-temperature vent fluids, geochemical budgets of the oceans, and the incidence of hydrothermal plumes, all arrive at similar estimates of ˜1,000 active vent sites along the mid-ocean ridges. As many as 500 additional vent sites may be located on submarine volcanic arcs and in back-arc basins, for a total of ˜1,500 sites. However, an analysis of the spatial distribution of known deposits, both on the mid-ocean ridges and in subduction-related environments, suggests that this is likely a maximum and that the total number of significant SMS occurrences along the neovolcanic zones of the world’s oceans is closer to ˜900. If the size distribution of the known deposits is representative of what remains to be discovered, then the total tonnage of SMS, excluding the Red Sea, is expected to be on the order of 600 Mt (˜1,000 deposits with a minimum size of 100 t and a maximum size of 10 Mt). The total contained metal would be about 30 Mt, based on a grade of 5 wt percent combined Zn + Cu + Pb. This estimate is similar to the total discovered metal in Cenozoic VMS deposits on land. However, it does not include long extinct deposits that may be located far off-axis. If present-day rates of massive sulfide formation on the mid-ocean ridges and back-arc spreading centers are extrapolated to older crust, then significant tonnages of massive sulfide may be expected beneath off-axis sediments. In contrast to land-based exploration, where larger deposits are commonly discovered early in the exploration history of a VMS district, exploration of the modern sea floor has discovered a high proportion of small, widely spaced SMS deposits. Large, inactive deposits are more difficult to identify by current exploration methods but may exist in isolated areas that have yet to be fully explored, such as in heavily sedimented back-arc rifts. This raises the possibility of a dramatically different resource future for SMS if one or more large deposits or “districts” are discovered that contain a high proportion of the total metal.