The HST science program is comprised of two principal components: the General Observer (GO) and the Guaranteed Time Observer (GTO) programs. A third component, the Archival Research (AR) program, will become increasingly important as time goes by. The GOs are selected by peer review of competitive proposals from the worldwide astronomical community, in a process managed by the STScI. The initial GO proposal deadline was October 1, 1988 and the corresponding review meetings took place during April 1989. The GTOs are members of the Investigation Definition and Observatory Scientist Teams who were competitively selected by NASA following an Announcement of Opportunity in 1977, to design and build the HST Science Instruments and participate in the project development, and who were awarded a decreasing fraction of the observing time during the initial two to three years of operations to conduct scientific observations with their instruments. The detailed GTO observing programs were received by the STScI on April 1, 1985, and had been revised a number of times since then.