The human brain contains 100 billion neurons and probably one thousand times more synapses. Such a system can be analyzed at different complexity levels, from cognitive functions to molecular structure of ion channels. However, it remains extremely difficult to establish links between these different levels. An alternative strategy relies on the use of much simpler animals that can be easily manipulated. In 1974, S. Brenner introduced the nematode Caenorhabditis elegans as a model system. This worm has a simple nervous system that only contains 302 neurons and about 7,000 synapses. Forward genetic screens are powerful tools to identify genes required for specific neuron functions and behaviors. Moreover, studies of mutant phenotypes can identify the function of a protein in the nervous system. The data that have been obtained in C. elegans demonstrate a fascinating conservation of the molecular and cellular biology of the neuron between worms and mammals through more than 550 million years of evolution.