The Technical Reference Manual is intended as a source of background information for users who want to know more about the technical/scientific contents of the HGSYSTEM modules used to model atmospheric dispersion of neutrally buoyant and heavier-than-air gases. The modules calculate release terms, evaporating liquid pools, jet dispersion, and heavy gas dispersion. The User's Guide contains all the information necessary to run HGSYSTEM and interpret results. The IBM-compatible software provided includes the source and executable codes of HGSYSTEM 3.0. Users require a minimum of a 386 processor, DOS 3.3, 4 MB RAM and 2.5 MB disk space. (Two binders are included.)