By analogy with the classical notions of density in the set N of natural numbers, one can introduce notions of density which are geared towards the multiplicative structure of N. Various combinatorial results involving additively large sets in (N,+) ( such as, for instance, Szemeredi's theorem on arithmetic progressions and its polynomial extensions) have natural analogs in the multiplicative semigroup (N,x). For example, muItiplicatively large sets in N contain arbitrarily long geometric progressions. One can show, that, somewhat surprisingly, multiplicatively large sets contain also arbitrarily long arithmetic progressions. The goal of this talk is to discuss some known results, open problems and conjectures pertaining to the interaction of the additive and multiplicative structures in N.