The morphology of dendrites is important for neuronal function and for proper connectivity within neuronal circuits. The often very complex shape of dendritic trees is brought about by the action of many different genes throughout development. Moreover, neuronal activity is often involved in refining synaptic connections and shaping dendrites. Detailed anatomical studies on two different types of interneurons in the Drosophila central nervous system are presented. They aim at improving our understanding of the interplay between genes and neuronal activity during dendrite differentiation using the fruit fly as a model.