Correlated neuronal activity is instrumental in the formation of networks, but its emergence during maturation is poorly understood. In my talk I will present how the development of population coherence from embryonic to postnatal stages in the hippocampus can be followed by multibeam two-photon calcium microscopy combined with targeted electrophysiological recordings. In particular I will describe a new form of immature correlated activity (SPAs : Synchronous Plateau Assemblies) that synchronizes at birth small cell assemblies coupled by gap junctions. These neuronal assemblies generate synchronous nonsynaptic calcium plateaus associated to recurrent burst discharges. I will present the mechanisms responsible for the generation of SPAs. I will demonstrate how, in the developing hippocampus, delivery is an important signal that triggers the first coherent activity pattern, which is silenced by the emergence of synaptic transmission.