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 both safe and modular: each box is described by a small set of inference rules and is implemented as an OCaml functor.