Caenorhabditis elegans is a small (~1mm long) nematode worm (roundworm) in the family Rhabditidae. It is cosmopolitan in distribution, with reproductive stages found most reliably in rotting fruits and surrounding soil. When food is plentiful, a fertilized egg completes embryogenesis, passes through four larval stages, and attains reproductive maturity in only three days at room temperature. As with most terrestrial nematodes, under stressful conditions an alternative third larval stage specialized for dispersal, the dauer larva, may be formed. In soil, C. elegans is often found in the dauer form. The reproductive mode of C. elegans involves a mixture of self-fertile hermaphrodites and males (this system was derived relatively recently from an ancestral male/female system). The transparency, anatomical simplicity, rapid development, and mix of outcrossing and selfing in C. elegans led American nematologist Ellsworth Dougherty and British molecular geneticist Sydney Brenner to champion this species as a model organism for basic biological research beginning in the 1970s. By the early 1980's, Brenner and colleagues had carried out pioneering studies investigating the invariant cell lineages, neuroanatomy, and aspects of the genome of C. elegans. This rich body of work garnered the attention of many more researchers and quickly led to C. elegans becoming one of the most widely studied laboratory organisms in the fields of genetics, cell biology, development, aging, evolution, and neuroscience. In 1998, C. elegans became the first animal to have its entire genome sequence determined and it remains at the forefront of functional genomics.