Refinement is a crucial process during final stages of neuronal circuit formation. The coarse map laid out early during development is refined by an interplay between cell intrinsic programs, external factors and neuronal activity. In many cases, it is closely related to emergence of function in the circuit. For example, refinement of the neuronal processes in mammalian visual circuit requires light and light deprivation can abrogate refinement. We followed refinement in Drosophila at a single cell resolution in a genetically tractable system. Further studies will aim at analyzing (i) generality of the mechanism and (ii) cell-biological basis of refinement.