Astrocytes communicate with synapses by means of intracellular calcium ([Ca²⁺]_i) elevations, but local calcium dynamics in astrocytic processes have never been thoroughly investigated. By taking advantage of high-resolution two-photon microscopy, we identify the characteristics of local astrocyte calcium activity in the adult mouse hippocampus. Astrocytic processes showed intense activity, triggered by physiological transmission at neighboring synapses. They encoded synchronous synaptic events generated by sparse action potentials into robust regional (∼12 μm) [Ca²⁺]_i elevations. Unexpectedly, they also sensed spontaneous synaptic events, producing highly confined (∼4 μm), fast (millisecond-scale) miniature Ca²⁺ responses. This Ca²⁺ activity in astrocytic processes is generated through GTP- and inositol-1,4,5-trisphosphate–dependent signaling and is relevant for basal synaptic function. Thus, buffering astrocyte [Ca²⁺]_i or blocking a receptor mediating local astrocyte Ca²⁺ signals decreased synaptic transmission reliability in minimal stimulation experiments. These data provide direct evidence that astrocytes are integrated in local synaptic functioning in adult brain.