Recent studies have identified various forms of active dendritic signals that may contribute to neuronal integration. One of the most remarkable findings is the demonstration of highly localized Ca2+ transients that are limited to small dendritic segments and even to single dendritic spines. In addition, through use of the powerful two-photon excitation imaging technique, it has been possible to reveal the existence of dendritic Ca2+ signals under in vivo conditions. Finally, active backpropagation of action potentials into dendrites has been shown to boost dendritic Ca2+ signals supralinearly and, thus, to contribute to the induction of long-term potentiation.