BACKGROUND Neurobiological research indicates that the adult central nervous system is more plastic than previously believed. This article reviews structural plasticity in the adult central nervous system. METHODS This review is based on literature retrieved from PubMed using the search terms "ltp AND learning", "synaptic AND structural plasticity", "adult AND neurogenesis", "pathology AND neurogenesis" and "MRI AND cortical plasticity". CONCLUSIONS Animal studies indicate that a minority of synapses turn over in the adult central nervous system. Learning is associated with increased synaptogenesis. In the adult human brain neurogenesis occurs in the hippocampus and in the olfactory bulb. Adult neurogenesis may contribute to some types of hippocampal-dependent learning, as well as to olfactory function. Increased neurogenesis in ischemic stroke and Huntington's disease indicates an endogenous regenerative response. Enhancing neurogenesis may represent a novel therapeutic strategy. In the adult human brain learning and medication can induce changes in grey matter detectable with MR-based morphometry. Emerging knowledge of neuroplasticity will have implications for theories of learning and treatment of disorders in the central nervous system.