BISIMULATOR: A modular tool for on-the-fly equivalence checking. The equivalence checking problem consists in verifying that a system (e.g., a protocol) matches its abstract specification (e.g., a service) by comparing their Labeled Transition Systems (Ltss) modulo a given equivalence relation. Two approaches are traditionally used to perform equivalence checking: global verification requires to construct the two Ltss before comparison, whereas local (or on-the-fly) verification allows to explore them incrementally during comparison. The latter approach is able to detect errors even in prohibitively large systems, and therefore reveals more effective in combating state explosion.