The HEASARC FITS Working Group, also known as the OGIP (Office of Guest Investigator Programs) FITS Working Group, has promoted multi-mission standards for the format of FITS data files in high-energy astrophysics. Its main activities took place in the mid-1990s, when it produced a number of documents and recommendations concerning the format of FITS files. Several of these recommendations have subsequently been incorporated into the FITS Standard format definition document.