Recent advances in whole genome analyses made possible by next-generation DNA sequencing, high-density array comparative genome hybridization (aCGH), and other technologies have made it apparent that cancers harbor numerous genomic changes. However, without functional correlation or validation, it has proven difficult to determine which genetic changes are necessary or sufficient to produce cancer. Thus, it is still necessary to perform unbiased functional studies using model organisms to help interpret the results of whole genome analyses of human tumors. To this end, a Sleeping Beauty (SB) transposon-based mutagenesis technology was developed to identify genes that, when mutated, can cause cancer. Herein a detailed methodology to initiate and carry out an SB transposon mutagenesis screen is described. Although this system might be used to identify genes involved with many cellular phenotypes, it has been primarily implemented for cancer. Thus, SB transposon somatic cell screens for cancer development are highlighted. Curr. Protoc. Mouse Biol. 1:347-368 © 2011 by John Wiley & Sons, Inc.