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This thesis describes a genetic and molecular study of programmed cell death in the nematode Caenorhabditis elegans. Programmed cell death is a process common to all metazoans examined, and plays an important role in development and disease. Work on C. elegans has demonstrated that in this organism programmed cell death also constitutes a major cell fate. Morphologically, programmed cell deaths in C. elegans are similar to programmed cell deaths found in other organisms, including mammals. These similarities can be demonstrated both on the light microscope and the electron microscope levels, and have suggested that the underlying molecular mechanisms of programmed cell death are similar between worm and man. To characterize the molecular components of cell death in C. elegans we have analyzed the cell death gene ced-3. Mutations in ced-3 abolish all programmed cell deaths, suggesting that this gene is necessary for the proper execution of the death program. We have shown that this gene encodes a cysteine protease of the Interleukin-1f converting enzyme (ICE) class and together ced-3 and ICE define a novel family o