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Astrocytes exhibit spontaneous calcium fluctuations. These activities have not been captured by large-scale recordings, and little is known about their collective dynamics. In situ and in vivo calcium imaging from hundreds (up to 2195) of astrocytes in the mouse hippocampus and neocortex revealed that neighboring astrocytes spontaneously exhibited synchronous calcium elevations and formed locally correlated cell groups (“clusters” of 2 to 5 astrocytes within a diameter of 81 ± 45 μm). Cluster activity accounted for approximately 10% of the astrocytic calcium events, and 44% of the clusters appeared repetitively during our observation period of 60 min. Astrocytic clusters emerged through metabotropic glutamate receptor activation, independently of neuronal activity. Neurons were depolarized by 1.5 mV when clusters appeared near their dendrites. This depolarization was mediated by non–N-methyl-D-aspartate (NMDA) glutamate receptor channels and was replicated by calcium uncaging activation of multiple astrocytes. Importantly, the activation of single astrocytes alone could not depolarize neurons but readily elicited NMDA-dependent slow inward currents in depolarized neurons. Thus, these novel ensemble dynamics of astrocytes, which cannot be captured by conventional small-scale imaging techniques, play a different role in neuronal modulation than does the sporadic activity of single astrocytes.