Alt-Ergo is an automated theorem prover implemented in OCaml. It is based on CC(X) — a congruence closure algorithm parameterized by an equational theory X. This algorithm is reminiscent of the Shostak algorithm. Currently CC(X) is instantiated by the theory of linear arithmetics. Alt-Ergo also contains a home made SAT-solver and an instantiation mechanism by which it fully supports quantifiers.