The neurotrophins are best known for their ability to support neuronal survival and differentiation, but a role in synapse formation and plasticity has recently emerged. For central neurons, brain-derived neurotrophic factor can increase the number of excitatory and inhibitory synapses by regulating axonal morphology or by directly promoting synapse formation. In addition, neurotrophins promote the maturation and stabilization of the cellular and molecular components that are responsible for neurotransmitter release, and this ultimately leads to an increase in the number of functional synapses. These long-term structural and molecular changes are likely to be crucial not only during development, but also during synaptic plasticity in the adult.