STSDAS is a system of some 750,000 lines of code and documentation designed for the calibration and analysis of data from the Hubble Space Telescope. At its inception in 1981, when it was known simply as ‘SDAS’, the system was dependent on the VAX VMS operating system and architecture and was limited in scope to HST data analysis, excluding calibration. As it was realized that astronomers would want to be able to do their own calibrations of HST data, and that it was not cost-effective, either for end-users or for the software development effort, to have a system dependent on a proprietary operating system, the scope and objectives of the system were modified. The most fundamental change was the decision to fully layer the STSDAS software on the Image Reduction and Analysis Facility -IRAF – from the National Optical Astronomy Observatories. For a review of the historical aspects of the development of STSDAS, please see Hanisch (1989).