't Hooft has made several crucial contributions to the Standard Model of particle physics. In 1967 Salam and Weinberg showed how to unify the weak and electromagnetic interactions - two of the four fundamental forces of nature - into a single electroweak force. In 1971 't Hooft showed that the electroweak force was renormalizable - in other words, that the infinities in the theory could be removed. 't Hooft has also made major contributions to our understanding of quantum chromodynamics, the theory that describes the strong nuclear force, and his work has had a major influence on string theory - currently the leading candidate for a theory that will unify all four forces.