Yang–Mills theory
In 1971, Gerardus 't Hooft, who was completing his PhD under the supervision of Martinus J. G. Veltman at Utrecht University, renormalized Yang–Mills theory in accordance with Veltman's suggestion that this was possible. They showed that if the symmetries of Yang–Mills theory were to be broken according to the method suggested by Robert Brout, François Englert, Peter W. Higgs, Gerald Guralnik, C. R. Hagen and Tom Kibble then Yang–Mills theory is indeed renormalizable. Renormalization of Yang–Mills theory is one of the biggest achievements of twentieth century physics. Gerardus 't Hooft and Martinus J. G. Veltman were awarded the Nobel Prize in Physics in 1999 for this work.