Special Topic Issue: Brain, Behavior and Evolution 2006, Vol. 68, No. 3 This special issue reviews current knowledge about the evolution of several key neurotrophic factors, including the neurotrophins and the GDNF family, as well as EGF receptor ligands. Leading researchers from six countries provide concise updates on the origin and evolution of neurotrophic factors and their receptors. The authors trace major developments in the structure and function of neurotrophic factors during the course of 800 million years of evolution, from prechordates to mammals. This collection of papers presents the first systematic and comprehensive survey of the origins and diversification of neurotrophic factors and their essential roles not only in ontogeny but also in phylogeny of the brain and behavior in both invertebrates and vertebrates. The contributions help achieve a true understanding of neurotrophic factors and their roles in brain evolution. This issue is a valuable source of information for developmental biologists, comparative and evolutionary biologists, molecular biologists, geneticists, and researchers interested in signaling systems, trophic factor biology, neuropharmacology, brain plasticity, and brain repair.