The generation of genetically modified animals using N-ethyl-N-nitrosourea (ENU) mutagenesis is a fast and highly effective method. The technique is based on treating male animals with the supermutagen ENU, which randomly introduces mutations in the spermatogonial stem cells. By breeding these animals with untreated females, an F1 population is generated in which each individual carries unique random ENU-induced mutations, which can be retrieved using either genotype-driven or phenotype-driven approaches. No complicated cell culturing techniques are required and since no foreign DNA is introduced, the mutant animals that are generated are not transgenic. Here, we describe the detailed protocols for ENU mutagenesis and for mutant retrieval.