The discovery of RNA interference (RNAi) has revolutionized genetic analysis in mammalian cells. Loss-of-function RNAi screens enable rapid, functional annotation of the genome. Of the various RNAi approaches, pooled shRNA libraries have received considerable attention because of their versatility. A number of genome-wide shRNA libraries have been constructed against the human and mouse genomes, and these libraries can be readily applied to a variety of screens to interrogate the function of human and mouse genes in an unbiased fashion. We provide an introduction to the technical aspects of using pooled shRNA libraries for genetic screens.