Brain function depends on the creation and maintenance of neural circuits with precisely defined patterns of connectivity.  We use the nematode, C. elegans, a model organism with a simple nervous system and powerful genetic tools to identify molecules that govern these events. Our work exploits a comprehensive wiring diagram that defines the anatomy and connectivity all 302 neurons in the C. elegans nervous system. We use a combination of mutant analysis and neuron-specific expression profiling to reveal genes that define the architecture of this network.