Traditionally, synchronous firing of neurons has been considered to be an epiphenomenon of neuronal networks, reflecting particular properties of circuitry, but having no functional relevance. In the past few years, an alternative view has been advocated, which suggests that temporal correlations serve a role in information processing by expressing relations among the responses of distributed neurons. This hypothesis has received experimental support from recent in vivo studies performed on the sensory systems of a variety of species. These results support earlier proposals that correlated activity might have an important function in sensory-motor integration and memory.