Overlapping decompositions and expansions of discrete-event systems (DESs) modeled by automata or formal languages are considered. Inclusion principle for such systems is defined. A decentralized supervisory controller design approach is then introduced. To apply the proposed approach, the automaton of the given DES is first decomposed overlappingly and expanded to obtain disjoint subautomata. A controller is then designed for each disjoint subautomaton. These controllers are then combined to obtain a controller for the expanded DES. In the final phase, a controller for the original DES is obtained from the controller determined for the expanded DES. It is shown that, for large-scale DESs, the proposed approach requires very little computation to design a controller compared to a centralized design approach.