Structured Circuit Semantics (SCS) is a formal semantics of robotic languages that can be used to explicitly represent the control behavior of various robotic control systems. Our approach to the design of SCS has been to incorporate the most essential features of other reactive plan execution systems in a compact, yet rich formal semantics. A formal specification of a reactive plan is essential for us to be able to generate a plan, reason about it, and communicate it among possibly multiple agents. Major design decisions include how to handle continuous actions and how to represent various agent behaviors such as nondeterministic behavior, best-first behavior, and persistent behavior. Metalevel reasoning and multiple threads of execution are also important design factors. Currently our effort is directed toward a semantics of multiagent robotic languages.