Genetic and biochemical analyses in model systems such as the fruitfly, Drosophila melanogaster, have successfully identified several genes that play key regulatory roles in fundamental cellular and developmental processes. However, the analyses of the complete genome sequences of Drosophila, as well as of humans, now reveal that traditional methods have ascribed functions to only a fraction of the total predicted genes. Thus, the roles for many, as yet unidentified genes, in normal development and cancer remain to be discovered. The challenge presented by the various large-scale genome projects is how to derive biologically relevant information from the raw sequences. The past few years have witnessed a rapid growth in the development and implementation high-throughput screening (HTS) technologies that researchers are now using to discover “ gene-function ” in an unbiased, systematic, and time-efficient manner. In fact one of the most promising functional genomic approach that has emerged in the past few years is based on RNA-interference (RNAi), in which the introduction of double-stranded RNA (dsRNA) into cells or whole organisms has been shown to be an effective tool to suppress endogenous gene expression. The RNAi technology has made it feasible to query the function of every gene in the genome for their potential function in a given cell-biological process using cell-based assays. This chapter discusses the application, advantages, and limitations of this powerful technology in the identification of novel modulators of cell-signaling pathways as well as its future scope and utility in designing more efficient genome-scale screens.