Mental processes are known to activate distributed networks of specialized neural structures. Neuroimaging techniques provide more and more precise pictures of these networks in different sensory and cognitive situations but the neural mechanisms underlying the network dynamics has been much less explored. It has been proposed that the co-operation within or between brain areas involved in sensory and cognitive processes could be based on the dynamic synchronization of the underlying neural populations in an oscillatory mode (in the beta and gamma ranges). This hypothesis has been supported at different levels, with unit and local field potential recordings in animal studies, and at intermediate (intracranial EEG) and more macroscopic levels (scalp EEG/MEG) in humans. We will review these human findings showing a functional role of beta/gamma oscillations in various mental processes related to perception, attention and memory. The contribution of these different levels of recording to investigate local- versus large-scale cortical synchronization will also be discussed.