Brout and Englert, Higgs, and Gerald Guralnik, C. R. Hagen, and Tom Kibble introduced as agent of the vacuum structure a scalar field (most often called the Higgs field) which many physicists view as the agent responsible for the masses of fundamental particles. Brout and Englert also showed that the mechanism may remain valid if the scalar field is replaced by a more structured agent such as a fermion condensate. Their approach led them to conjecture that the theory is renormalizable. The eventual proof of renormalizability, a major achievement of twentieth century physics, is due to Gerardus 't Hooft and Martinus Veltman who were awarded the 1999 Nobel Prize for this work. The Brout–Englert–Higgs–Guralnik–Hagen–Kibble mechanism is the building stone of the electroweak theory of elementary particles and laid the foundation of a unified view of the basic laws of nature.