In physics, the Green-Schwarz mechanism is the main discovery that started the first superstring revolution in superstring theory in 1984. Michael Green and John Schwarz realized that the anomaly (physics) in type I string theory with the gauge group SO(32) cancels because of an extra "classical" contribution.

In the original calculation, gauge anomalies, mixed anomalies, and gravitational anomalies were expected to arise from a hexagon Feynman diagram. For the special choice of the gauge group SO(32) or E8 x E8, however, the anomaly factorizes and may be cancelled by a tree diagram. In string theory, this indeed occurs. The tree diagram describes the exchange of a virtual quantum of the B-field. It is somewhat counterintuitive to see that a tree diagram cancels a one-loop diagram, but in reality, both of these diagrams arise as one-loop diagrams in superstring theory in which the anomaly cancellation is more transparent.