Experience alters brain structure. Structural plasticity reveals that brain function is encoded in generative changes to cells that compete with destructive processes driving neurodegeneration. At an adult critical period, experience increases fiber number and brain size in Drosophila. In mammals, the neurotrophins are the key growth factors that link structure and function in the brain. They regulate neuronal survival and connectivity, synaptogenesis and synaptic function, via their tyrosine kinase Trk and p75 receptors. In this talk, I will review the Drosophila neurotrophin system. We showed that Drosophila neurotrophins work via Toll receptors and kinase-less Trk-like receptors to regulate the same processes as in mammals, but via novel molecular mechanisms. Toll and Toll-like receptors in mammals were best known for their functions in innate immunity. I will present our findings on the involvement of Toll receptors in structural brain plasticity. Through their topographic distribution in the brain and their ability to switch between alternative signalling outcomes, Tolls can translate diverse sensory experience into structural change.