Model checking has found a role in the engineering of reactive systems. However, model checkers are still strongly limited by the size of the system description they can check. Here we present a technique in which a system is simplified prior to model checking by the application of abstraction rules. The rules can greatly reduce the state space of a system description and help in understanding why a system satisfies a property. We illustrate the use of the technique on examples, including Dekker's mutual exclusion algorithm.