Renormalization : an introduction This book provides a simple but mathematically rigorous introduction to perturbative renormalization. It can be read by students from the third year on and will leads them to a level where they can start reading the current research literature. The book gives a thorough introduction to the field theoretic techniques required for renormalization. Special effort has been devoted to simplifying combinatorial proofs as much as possible by using generating function techniques. A renormalization group differential equation is used to study the flow of Wilsons effective action. This differential equation technique developed during the last few years is used to give simple but complete proofs of renormalizability theorems. The book contains an introduction to renormalization in models of solid state physics