PetDB, the Petrological Database, is a web-based data management system that provides on-line access to geochemical and petrological data. PetDB is a global synthesis of chemical, isotopic, and mineralogical data for rocks, minerals, and melt inclusions. PetDB's current content focuses on data for igneous and metamorphic rocks from the ocean floor, specifically mid-ocean ridge basalts and abyssal peridotites and xenolith samples from the Earth's mantle and lower crust. PetDB is maintained and continuously updated as part of the EarthChem data collections.