In this paper, we develop a semantic model, constraint nets (CN), for hybrid systems. CN captures the most general structure of dynamic systems so that systems with discrete and continuous time, discrete and continuous variables, and asynchronous as well as synchronous event structures, can be modeled in a unitary framework. Using aggregation operators, a system can be modeled hierarchically in CN; therefore, the dynamics of the environment as well as the dynamics of the plant and the dynamics of the controller can be modeled individually and then integrated. Based on abstract algebra and topology, CN supports multiple levels of abstraction, so that a system can be analyzed at different levels of detail. CN also provides a rigorous formal programming semantics for the design of hybrid real-time embedded systems.