In March 1986 a research borehole, called the ''State 2--4,'' reached a depth of 3.22 km in the Salton Sea geothermal system of southern California. This was part of the Salton Sea Scientific Drilling Project (SSSDP), the first major (i.e., multimillion dollar) research drilling project in the U.S. Continental Scientific Drilling Program. The principal goals of the project were to investigate the physical and chemical processes of a high-temperature, high-salinity, magmatically driven hydrothermal system. The borehole encountered temperatures of up to 355 ¿C and produced metal-rich, alkali chloride brines containing 25 wt % of total dissolved solids. The rocks penetrated exhibit metamorphism and ore genesis in action. They show a progressive transition from unconsolidated lacustrine and deltaic sediments to hornfelses, with lower amphibolite facies mineralogy, accompanied by pervasive veins containing iron, copper, lead, and zinc ore minerals. The SSSDP included an intensive program of rock and fluid sampling, flow testing, and downhole logging and scientific measurement. The purpose of this paper is to introduce this special section of the Journal of Geophysical Research on the SSSDP, to describe briefly the background of the project and the drilling and testing of the borehole, to summarize the initial scientific results, and to discuss how the lessons learned are applicable to future scientific drilling projects. ¿ American Geophysical Union 1988