This paper provides simulation practItIoners and interested simulation consumers with a grounding in how discrete-event simulation software works. Topics include discrete-event systems and modeling: entities, resources and operations ~ simulation runs ~ entity states~ entity lists ~ and entity-list management. The implementation of these generic ideas in SIMAN, ProModel, and GPSS/H is described. The paper concludes with several examples of "why it matters " for modelers to know in fine detail how their simulation software works. (This paper is an updated version of an identically named paper appearing in the Proceedings ofthe 1995 Winter Simulation Conference, pp. 110-117.