The ability to integrate sophisticated planning techniques with reactive execution systems is critical for non-trivial applications. Merging these two technologies is difficult because the forms of knowledge and reasoning that they employ differ substantially. The ACT formalism is a language for representing the knowledge required to support both the generation of complex plans and reactive execution of those plans in dynamic environments. A design goal of ACT was its adequacy for practical applications. ACT has been used as the interlingua in an implemented system that links a previously implemented planner with a previously implemented executor. This system has been used in several applications, including robot control and military operations, thus attesting to its expressive and computational adequacy.