EFOM-ENV (Energy Flow and Optimization Model - ENVironment) is a linear programming model which models national energy systems. It improves the flexibility the Netherlands Energy Research Foundation specified EFOM-ENV in GAMS. GAMS (General Algebraic Modelling System) is a modelling language, especially developed for large and complicated models. GAMS can also be applied on a work station and personal computer. The EFOM-ENV/GAMS interface checks the data at input time, and will not accept illegalities. The basis of the interface is a form, accomodated to EFOM-ENV, to be completed by the user. The form contains all the necessary data of a PROCESS, the most disaggregated level of an energy conversion technology. Also the data of related UNITS, energy carriers and abatement technologies can be entered on overlayed forms, which pop up on request or in some cases are prompted when data is needed. The EFOM-ENV/GAMS interface is a self-explanatory, menu-driven program. In this manual a detailed description is given of every possible input with its restrictions.