Techniques and tools for achieving improved test coverage in a finite program state space are described, such as a technique for selecting a set of predicates, calculating a set of possible predicate values, calculating a subset of the set of possible predicate values, and generating a test for the computer program based at least in part on the subset. The subset comprises an approximation (e.g., an under-approximation) of reachable states in the program. A superset of the set of possible predicate values also can be calculated; the superset comprises an over-approximation of the reachable states in the program. In another aspect, a Boolean abstraction of a program is generated, reachability analysis is performed based at least in part on the Boolean abstraction, and symbolic execution is performed to generate test data. The reachability analysis can include computing lower and/or upper bounds of reachable observable states.