For decades glial cells were considered relatively passive supporters of nerve function. It was accepted that they had important roles to play in the structural, metabolic and trophic support of neurons; however, it is now also known that glial cells are active in the processes of synaptic transmission and information processing. Electrophysiological and optical imaging studies demonstrate that glia not only receive coded information concerning neighbouring synaptic events, but also integrate the exchange of rapid signals with nerve cells and exert modulatory influences at the local and long-range levels. As the specific mechanisms and functional consequences of the glial-synaptic relationship becomes clearer, so the broader relevance of these findings can be understood. The Tripartite Synapse: Glia in Synaptic Transmission introduces the concept of glial cells as active elements of the synapse and the nervous circuitry, describes the experimental evidence supporting this and suggests a revised view of brain activity based on this integrated network of interactive neurons and glia. This is a timely and international review of an exciting new field, and will be of interest to neuroscientists, neurobiologists and cell biologists