Describes the major design features of ECSS, a programming language for simulating computer systems as an aid to design and evaluation. In its prototype version, ECSS is implemented as a translator into SIMSCRIPT II with extensions to add further capabilities. ECSS can model flow-oriented as well as discrete-event simulations, having added to SIMSCRIPT the SIMULA-like process, which is both an entity and an event subroutine, and the HOLD UNTIL and WHEN TRUE statements. The emphasis throughout is on ease of use, freedom in modeling, and on minimizing programming and debugging time. Standard features can be simulated merely by requesting the facilities and inputting data; for nonstandard features, the user writes his own description in SIMSCRIPT and uses as much as he wishes of the ECSS facilities. Unused facilities do not affect execution. Implementation of the ECSS translator is now under way. 53 pp. Ref.