Studies of the nematode Caenorhabditis elegans have identified genetic and molecular mechanisms controlling temporal patterns of developmental events. Mutations in genes of the C. elegans heterochronic pathway cause altered temporal patterns of larval development, in which cells at certain larval stages execute cell division patterns or differentiation programs normally specific for other stages. The products of the heterochronic genes include transcriptional and translational regulators and two different cases of novel small translational regulatory RNAs. Other genes of the pathway encode evolutionarily conserved proteins, including a homolog of the Drosophila Period circadian timing regulator, and a member of the nuclear receptor family of proteins. These regulators interact with each other to elaborate stage-specific regulatory switches and act through downstream effectors to control the timing of cell-type-specific developmental events.