Alt-Ergo is an automatic theorem prover dedicated to program verification. Alt-Ergo is based on CC(X), a congruence closure algorithm parameterized by an equational theory X. Currently, CC(X) can be instantiated by the empty equational theory and by the linear arithmetics. Alt-Ergo contains also a home made SAT-solver and an instantiation mechanism. Alt-Ergo is compact, safe, and modular. Each component is described by a small set of inference rules and is implemented as an Ocaml functor. WWW: http://alt-ergo.lri.fr