The book explores a number of different features of congruence on universal algebras. It covers the subject from the early works of Maltsev to recent results. One can find a good amount of material concerning properties of blocks (in general and in varieties), relationship to quotients and subalgebras or local properties of congruence. The first two chapters explain basic notions and give quite a few examples of algebras. The study of congruence starts with the classical notions of permutability, distributivity, modularity and direct decomposability. Further chapters deal with the role of subalgebras, single blocks, constant terms or ideals. Special chapters are devoted to extension properties of congruence, regular and coherent algebras, local conditions and one-block congruence.