By combining electrophysiology and 2-photon laser scanning microscopy, we provided evidence for a definitive link between activity at glutamate synapses, the recruitment of astrocytes and scaling of synapses (Gordon et al., Neuron, 64(3):391-403, 2009). The observations reported here provide new insights into the regulation of central synapses by astrocytes and demonstrate that small domains on individual astrocytes can simultaneously scale all glutamatergic synapses on a neighbouring neuron. This work challenges a central tenet that plasticity occurs in a restricted, synapse specific fashion. That activation of a single astrocyte can influence multiple synapses on and across neurons in the absence of action potentials suggests that under some situations, many synapses may alter their efficacy rapidly, while at the same time preserving the relative weights between them. This would retain the previous specificity ingrained by Hebbian forms of plasticity and would also lead to a general enhancement of network output without comprising the neural circuit.