Abstract-Cultured neurons in vitro quickly connect to one another to establish a spontaneously active network within a week. The resulting neuronal network is characterized by a combination of excitatory and inhibitory integrate-and-fire units coupled through synaptic connections, 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 activity, avalanches, propagation of activity fronts, and synchronization. Here we describe some experimental results on spontaneous activity in cultures, and elaborate on theoretical models that describe its initiation and maintenance.