It has been proposed that learning is accomplished by the formation of cell assemblies and synchronous activity among the neurons of such a network. In a series of studies we have tested the hypothesis that oscillatory activity in the Gamma Band is a signature of different memory processes. In particular, the focus was directed on rapid perceptual learning, recognition memory, and repetition priming. Using multi-channel EEG the following results were found: (1) Retrieval processes in a perceptual learning paradigm led to an increase of Gamma power and phase synchrony. (2) Gamma Band activity is crucial not only during retrieval but also for the establishment of a valid memory trace. (3) Repetition priming of familiar objects resulted in repetition suppression in the Gamma Band, possibly related to a 'sharpening' in cortical representations. Repetition of unfamiliar objects led to repetition enhancement, which suggests that repetition effects depend on the presence or absence of pre-existing object representations. From this series of studies, it was concluded that memory retrieval/formation is indeed based on networks, which are established by neuronal synchrony amongst their elements. Induced Gamma Band responses and phase synchrony are a signature of activity within such a cell assembly in the human EEG.