Tectonic settings, compositions, and genesis of solutions of hydrothermal systems from the Pacific are discussed. In onshore (subaerial) hydrothermal systems, solutions of several different genetic types can cause base-metal sulfide mineralization due to high acidity or salinity. At least two types of offshore hydrothermal solutions forming such mineralization are known: those of chloride calcium-sodium composition (most common) and those of sulfate-chloride magnesium-sodium composition. The first type of solution is produced by water-rock interaction without considerable admixture of magmatic fluids from the known massive sulfide deposits of midoceanic, intracontinental, and back-arc rift zones. The second type of solution is strongly influenced by acidic fluids of volcanic origin, increasing their ability to leach and transport ore metals, and they may also form sulfide mineralization in marginal parts of the ocean. -Authors