We propose a new specification environment for system-level design called ECL. It combines the Esterel and C languages to provide a more versatile means for specifying heterogeneous designs. It can be viewed as the addition to C of explicit constructs from Esterel for waiting, concurrency and pre-emption, and thus makes these operations easier to specify and more apparent. An ECL specification is compiled into a reactive part (an extended finite state machine representing most of the ECL program), and a pure data looping part, thus nicely supporting a mix of control and data. The reactive part can be robustly estimated and synthesized to hardware or software, while the data looping part is implemented in software as specified.