In this thesis we construct an additive category whose objects are embedded graphs (or in particular knots) in the 3-sphere and where morphisms are formal linear combinations of 3-manifolds. Our definition of correspondences relies on the Alexander branched covering theorem [1], which shows that all compact oriented 3-manifolds can be realized as branched coverings of the 3-sphere, with branched locus an embedded (not necessarily connected) graph. The way in which a given 3-manifold is realized as a branched cover is highly not unique. It is precisely this lack of uniqueness that makes it possible to regard 3-manifolds as correspondences. In fact, we show that, by considering a 3-manifold M realized in two different ways as a covering of the 3-sphere as defining a correspondence between the branch loci of the two covering maps, we obtain a well defined associative composition of correspondences given by the fibered product.