The aim of the talk is to introduce level algebras: we'll give some examples coming from algebraic topology and then we'll explain what are Adem-Cartan operads. Adem-Cartan operads are operads defined from the operad Lev, the one defining level algebras. The main theorem is that the cohomology of any algebra over an Adem-Cartan operad is endowed with a structure of unstable algebra over the Steenrod algebra and with secondary cohomological operations.