Ever since the cell lineage of the nematode Caenorhabditis elegans was shown to be nearly invariant, investigators have tried to understand the mechanisms that control these precise patterns of cell divisions and cell fates. Important insights have come from analyzing the cells that form the hermaphrodite vulva, a specialized hypodermal passageway used for egg laying and sperm entry. Early experiments showed that the invariant pattern of vulval cell fates requires highly reproducible intercellular signals. This review describes recent experiments that have begun to characterize molecules that mediate these signals and explore the relationships between different signaling pathways. Many of these molecules and signaling pathways have been conserved during evolution suggesting mechanisms used to establish patterns of cell fates during vulval development have also been conserved.