The nematode worm Caenorhabditis elegans is a major model organism for studies in genetics, cell biology, neurobiology, and development. The use of C. elegans as a model organism began in the 1950s. The cellular basis of the development and neuroanatomy of C. elegans have been described in detail. Studies of C. elegans have elucidated the genetic basis of apoptotic cell death, axon guidance, aging, and RNA interference. The C. elegans genome is one of the smallest known for any animal and was the first animal genome to be completely sequenced. The number of genes in the worm genome is comparable to that of other larger animals, suggesting that although C. elegans is small, it has comparable biological complexity to other animals. The functions of most C. elegans genes are not known. C. elegans is well suited to both forward and reverse genetic manipulations of the genome. Postgenomic studies are continuing to explore the biological complexity of C. elegans .