Prompted by an increasing interest in accelerated heavy ions a study group at the University of Heidelberg started in the early 1960s a design study for a heavy ion accelerator. The aim was to work out an accelerator concept which would make it possible to accelerate atomic ions of any mass, i.e. up to uranium, to energies at which nuclear reactions could be induced in any specified target atoms. As a result of this study a radio-frequency linear accelerator (Unilac) was proposed. It was built as the central facility of the GSI in the years from 1972 to 1975. Several upgrades of the Unilac improved the performance and flexibility over the years, allowing pulsed switching between up to three different ion beams since 2000. From 1986 to 1990 the GSI accelerator facility has been extended by an 18Tm synchrotron (SIS18), with the Unilac as injector, and by an Experimental Storage Ring (ESR). With this facility both a step towards higher energies and into the new field of cooling highly charged heavy ions was made. Electron cooled beams of highly charged heavy ions in the ESR opened new fields for experiments in atomic and nuclear physics. Later a stochastic cooling system was installed for pre-cooling of “hot” radioactive fragment beams. Mid 1990s the accelerator control system was upgraded for the requirements of the cancer therapy program, which started in 1997.