When the performance of a team of agents exceeds our expectations or fall short of them, we often explain this by saying that there was some synergy in the team---either positive (the team exceeded our expectations) or negative (they fell short). Our aim in this article is to develop a formal and principled way of measuring synergies, both positive and negative. Using characteristic function cooperative games as our underlying model, we present a formal measure of synergy, based on the idea that a synergy is exhibited when the performance of a team deviates from the norm. We then show that our synergy value is the only possible such measure that satisfies certain intuitive properties. We then investigate some alternative characterisations of this measure.