Plastic changes in the connectivity between neurons underly the adaptive information processing of the central nervous system. The shortest forms of such plasticity are synaptic depression and facilitation, which happen on the subsecond time scale. New techniques allow to study synaptic transmission at unprecedented resolution and to dissect its various components, such as presynaptic Ca++ currents, Ca++ signals, and transmitter stores. Precise knowledge on the components, which are altered during plastic changes, is essential for understanding these important mechanisms.