First we propose a new way of combining two polynomial projectors on spaces of functions of few variables to obtain a polynomial projector on a space of functions of many variables. We present various algebraic properties of our construction and study the main approximation properties of the new projectors. We prove in the second chapter that the Newton product of efficient polynomial projectors is still efficient. Various polynomial approximation theorems are established involving Newton product projectors on spaces of holomorphic functions on a neighborhood of a regular compact set, on spaces of entire functions of given growth and on spaces of differentiable functions. Efficient explicit new projectors are presented. Finally we study the following problem : given a valid set X of interpolation points for Lagrange interpolation of degree d in n variables we study how many subsets of X can be chosen in order to obtain a valid set of interpolation points of degree d — 1. This leads to an estimate of the number of Newton structures for X which, in turngives the number of different unisolvent sets that can be obtainend by the process of interwinning defined in the first chapter.