The present unit is the seventh unit of the walk The Axioms of Zermelo and Fraenkel. We will explain the last of the axioms of Zermelo and Fraenkel, namely the axiom of infinity. It will allows us to deduce the axioms of Peano from the axioms of Zermelo and Fraenkel. As a consequence we will be able to formally introduce the natural numbers in Unit The Natural Numbers and the Principle of Induction.