Cultured neurons in vitro quickly connect to one another to establish a spontaneously active network within a week. The resulting neuronal network is char- acterized by a combination of excitatory and inhibitory integrate–and–fire units coupled through synaptic connec- tions, and that interact in a highly nonlinear manner. The nonlinear behavior emerges from the dynamics of both the neurons’ membrane potential and synaptic transmission, together with intrinsic biological noise. These ingredients give rise to a rich repertoire of phenomena that are still poorly understood, including periodic spontaneous activ- ity, avalanches, propagation of activity fronts, and synchro- nization. Here we describe some experimental results on spontaneous activity in cultures, and elaborate on theoreti- cal models that describe its initiation and maintenance

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