In our papers dealing with the reduction of singularities of an algebraic surface (see [8, ll]), we were forced to devote a good deal of space to certain properties of birational correspondences for which we could find no general proofs in the literature. These properties were of a general character and therefore could not be regarded as part of the reduction proof proper, although they did play an auxiliary role in the proof. A similar situation arose in our reduction proof for three-dimensional varieties (not yet published), but in this case the amount of preliminary general material on birational correspondences used in the proof was even larger and was out of proportion to the length of the reduction proof proper. It thus became increasingly clear that the procedure of treating general questions of birational correspondences only as and when these questions come up in connection with various steps of the reduction process, could no longer be followed in the case of higher varieties. Instead it seemed necessary-and also worthwhile for its own saketo develop systematically in a separate paper the fundamental concepts and theorems of the theory of birational correspondences, and to do this in as general a fashion as possible. This we propose to do in the present paper. We deal here with algebraic varieties, with or without singularities, over an arbitrary ground field (of characteristic zero or p). It is difficult to say which of our results are entirely novel and which are not. Since many of the results hold only for normal varieties, they would appear to be novel inasmuch as our concept of a normal variety is new. On the other hand, most of our results were known for nonsingular models. It is perhaps correct to say that the novelty of the present investigation consists in showing that most of the known properties of birational correspondences between nonsingular varieties remain true more generally for normal varieties.