L'alg`ebre de Steenrod, modules injectifs, et foncteurs polynomiaux Lionel Schwartz These notes come from talks made in Nantes in December 2001 at a session " Etat de la Recherche " of the french mathematic society. They are an introduction to the algebraic aspects of the theory of unstable modules over the Steenrod algebra and to the relations of the related category to functor categories. The Steenrod algebra is introduced using the additive group scheme. Reduced injective modules are described follwing the point of view of Campbell and Selick. Most of the material is classical, however there are new (at least in an accessible form) remarks concerning the odd prime case, as well as some new proofs of classical results, in particular the structure of Miller's algebra. The Adem relations are discussed following Bullett and MacDonald.