LIN-12/Notch proteins function as receptors for intercellular signals during development. Many aspects of LIN-12/Notch-mediated signaling have been elucidated through studies of cell–cell interactions that occur during Caenorhabditis elegans and Drosophila melanogaster development. The basic principles that operate in these lower organisms have also been shown to apply to vertebrates (for review, see Gridley 1997). Molecular features defined in lower organisms have also been shown to be conserved in vertebrates, including components of the signaling and signal transduction systems (for review, see Weinmaster 1997). The focus of this paper is on what has been learned about LIN-12/Notch signaling from invertebrates. First, a description of roles for LIN-12/Notch proteins in development is given, using different model cell fate decisions to illustrate various features. A discussion of the mechanism of LIN-12/Notch signal transduction follows, including new in vivo evidence that favors the direct participation of the intracellular domain of LIN-12/Notch proteins in regulating target gene expression. Finally, other influences on LIN-12/Notch activity are discussed, particularly protein turnover and protein processing.