Yang–Mills theory
In 1971, Gerardus 't Hooft, who was completing his PhD under the supervision of Veltman, renormalized Yang–Mills theory. They showed that if the symmetries of Yang–Mills theory were to be realized in the spontaneously broken mode, referred to as the Higgs mechanism, then Yang–Mills theory can be renormalized. Renormalization of Yang–Mills theory is a major achievement of twentieth century physics.