Since the discovery of nerve growth factor, there has been accumulating evidence that neurotrophins (NTs) mediate various biological responses of peripheral and central neurons. NTs have been traditionally studied as the regulating factors of neuronal survival and differentiation. Recent data indicate that NTs can modify neuronal plasticity by specific changes in pre- and post-synaptic functions. Whether the NT action is pre- or post-synaptic, however, remains to be controversy. Here we review the recent advances of NTs involved in synaptic plasticity, as well as the pre- and post-synaptic arguments. We also review the recent discovery that proneurotrophins and mature NTs have the differential ability to bind selective receptors and mediate distinctive biological actions.