Dendrite complexity determines the functional properties of neurons and the overall connectivity of neuronal circuits. The bone morphogenetic protein (BMP) family is known to regulate a myriad of developmental processes, but the extent to which different members of the family are involved in dendrite growth remains unclear. In this issue (p. 4751), Alun Davies and colleagues identify growth differentiation factor 5 (GDF5), a member of the BMP family, as a key regulator of dendrite growth and complexity in the pyramidal neurons of the developing hippocampus. Mice harbouring a mutation in Gdf5 showed dramatically reduced dendrite size and complexity. In vitro, exogenous GDF5 treatment was sufficient to increase elongation of the dendrites, but not the axons, of pyramidal cells derived from the developing mouse hippocampus. The authors further demonstrated that GDF5-mediated dendrite growth acts via the Smad signalling pathway and that GDF5-regulated HES5 expression is both necessary and sufficient for enhanced dendritic growth and complexity.