Genetic and molecular studies of development in Caenorhabditis elegans have identified regulators that appear to control pattern formation in the cellularized nematode embryo. Two genes, skn-1 and pie-1, are required for specifying the different identities of two sister blastomeres in a 4-cell embryo, called P2 and EMS. The skn-1 gene encodes a DNA binding protein that may control blastomere development by regulating transcription in EMS and its descendants. ABa and ABp, the other two sisters in a 4-cell embryo, are influenced to develop differently by cell signaling events that require the two genes apx-1 and glp-1. In this review, I summarize evidence that some or all of these genes may encode embryonic determinants of blastomere identity.