While changes in the efficacy of synaptic transmission are believed to represent the physiological bases of learning mechanisms, other recent studies have started to highlight the possibility that a structural reorganization of synaptic networks could also be involved. Morphological changes of the shape or size of dendritic spines or of the organization of postsynaptic densities have been described in several studies, as well as the growth and formation following stimulation of new protrusions. Confocal in vivo imaging experiments have further revealed that dendritic spines undergo a continuous turnover and replacement process that may vary as a function of development, but can be markedly enhanced by sensory activation or following brain damage. The implications of these new aspects of plasticity for learning and memory mechanisms are discussed.