We propose a unifying framework for model-based specification notations. Our framework captures the execution semantics that are common among model-based notations, and leaves the distinct elements to be defined by a set of parameters. The basic components of a specification are non-concurrent state-transition machines, which are combined by composition operators to form more complex, concurrent specifications. We define the step-semantics of these basic components in terms of an operational

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... antics template whose parameters specialize both the enabling of transitions and transitions' effects. We also provide the operational semantics of seven composition operators, defining each as the concurrent execution of components, with changes to their shared variables and events to reflect inter-component communication and synchronization; the definitions of these operators use the template parameters to preserve in composition notation-specific behaviour. By separating a notation's step-semantics from its composition and concurrency operators, we simplify the definitions of both. Our framework is sufficient to capture the semantics of basic transition systems, CSP, CCS, basic LOTOS, ESTELLE, a subset of SDL88, and a variety of statecharts notations. We believe that a description of a notation's semantics in our framework can be used as input to a tool that automatically generates formal analysis tools.