The latest generation of molecular-targeted cancer therapeutics has bolstered the notion that a better understanding of the networks governing cancer pathogenesis can be translated into substantial clinical benefits. However, functional annotation exists for only a small proportion of genes in the human genome, raising the likelihood that many cancer-relevant genes and potential drug targets await identification. Unbiased genetic screens in invertebrate organisms have provided substantial insightsinto signaling networks underlying many cellular and organismal processes. However, such approaches in mammalian cellshave been limited by the lack of genetic tools. The emergence of RNA interference (RNAi) as a mechanism to suppress geneexpression has revolutionized genetics in mammalian cells and has begun to facilitate decoding of gene functions on agenome scale. Here, we discuss the application of such RNAi-based genetic approaches to elucidating cancer-signaling networks and uncovering cancer vulnerabilities.