The Petrological Database of the Ocean Floor (PetDB) is a relational database for global geochemical data on igneous and metamorphic rocks generated at mid-ocean ridges including back-arc basins, young seamounts, and old oceanic crust, as well as ophiolites and terrestrial xenoliths from the mantle and lower crust and diamond geochemistry. These data are obtained by analyses of whole rock powders, volcanic glasses, and minerals by a wide range of techniques including mass spectrometry, atomic emission spectrometry, x-ray fluorescence spectrometry, and wet chemical analyses. Data are compiled from the scientific literature by PetDB data managers, and entered after methodical metadata review. Members of the scientific community can also suggest entry of specific data that has been entered into the EarthChem Library. PetDB is administered by the EarthChem group under the IEDA facility at LDEO headed by K. Lehnert. PetDB is supported by the U.S. National Science Foundation.