Single-nucleotide polymorphism (SNP) mapping is the easiest and most reliable way to map genes in Caenorhabditis elegans. SNPs are extremely dense and usually have no associated phenotype, making them ideal markers for mapping. SNP mapping has three steps. First, recombinant mutant animals are generated over a polymorphic strain (usually CB4856) using standard genetic techniques. Second, the genotype of these animals at SNP loci is determined using one of a variety of SNP detection technologies. Third, linkage between the mutant and one or more SNPs is used to position the mutant on the chromosome relative to the SNPs. This chapter presents a detailed procedure for generating recombinant animals, for assaying SNPs using restriction enzymes, and for analyzing mapping data.