SXL, a modeling language that describes system behavior rather that software structure, is discussed. Using a conventional state-transition framework, model behavior is determined by rules that define pre- and postconditions for each transition. Behavior is also specified by constraints (logical invariants) that are automatically enforced during the execution of the model. Rules and constraints are expressed solely in terms of entity-relationship structure and declarative logic; the language lacks machine-oriented data or control structures, and has no facilities for specifying or implementing software. Application of SXL is demonstrated by its translation of a simple behavioral description (a scenario from an actual requirements document) into an executable model. Comparisons are made to software- and specification-oriented methods to illustrate the tradeoffs resulting from SXL's restriction to simple behavioral modeling. A brief account is given of one software development group's experience with SXL.>