ABSTRACT The human brain forms about 100 trillion synapses during development. These synapses determine how neural circuits process information and perform computations, and when dysregulated, likely form the basis of many neurological and psychiatric diseases. Both autonomous genetic programs and spontaneous activity are thought to contribute to synaptogenesis. However, the precise role of spontaneous activity and the mechanisms underlying its generation are not well understood. Recent work has established that spontaneous activity accompanies the development of synapses in the Drosophila visual system. The fly brain was previously thought to develop synapses solely via preprogrammed genetic protocols, and this observation is the first evidence to suggest that spontaneous activity may also contribute to synaptogenesis in this system. Preliminary studies have characterized the natural history of this activity, its mechanisms of propagation, and its patterns at the level of individual cell types. Remarkably, cells that are synaptic partners in the adult show correlated activity patterns. These data suggest that spontaneous activity is a fundamental phenomenon of circuit assembly. The extensive genetic toolbox and mapped connectome in the fly visual system allow for molecular and cellular dissection of the relationship between spontaneous activity and synapse formation. The proposed research plan will use two- photon calcium imaging and genetic advantages in Drosophila to elucidate the role of spontaneous activity in synaptogenesis. Further studies will identify molecular and cellular mechanisms underlying the generation and propagation of spontaneous activity. These results will indicate how spontaneous activity influences synapse formation in the developing brain and will provide important insights into the mechanisms of this activity in the assembly of neural circuits.