Study of Caenorhabditis elegans embryonic development has been useful to dissect the molecular mechanisms controlling cell proliferation, cell polarization, cell differentiation, and morphogenic events also involved in embryogenesis in human (1, 2). The strength of this organism for developmental research consists in its amenability to large-scale genetic screening, its simple morphology, and transparency enabling study of developmental processes at a single cell level. Large-scale genetic screening targeting embryonic development in C. elegans is usually poorly sensitive and non-quantitative (3, 4). In this chapter we detail a novel approach enabling genetic dissection of C. elegans embryogenesis in a quantitative and semi-automated manner. This approach based on RNAi and flow cytometry enables the measurement of discrete embryonic lethal phenotypes and staging of arrested embryos.