Discovery of hydrothermal vents and seafloor massive sulfides (SMS) that contain metals of economic importance due to their high concentrations has generated significant interest among researchers as well as entrepreneurs as an alternative source that can be mined in future. This chapter provides a brief historical review of hydrothermal systems, the distribution, geological setting, morphology, composition, and age as well as formation and source of metals in SMS deposits. The chapter also looks at the criteria for recognition and exploration technologies for SMS deposits.