Vienna Standard Mean Ocean Water (VSMOW) is a water standard defining the isotopic composition of fresh water. It was promulgated by the International Atomic Energy Agency (based in Vienna) in 1968, and, since 1993, continues to be evaluated and studied by the IAEA along with the European Institute for Reference Materials and Measurements and the American National Institute of Standards and Technology. The standard includes both the established values of stable isotopes found in waters and calibration materials provided for standardization and interlaboratory comparisons of instruments used to measure these values in experimental materials.