Born in Den Helder, Netherlands, 't Hooft completed high school in The Hague in 1964. He studied physics and mathematics at the University of Utrecht, Netherlands, completing his undergraduate degree in 1966 and his Ph.D. degree in theoretical physics in 1972. From 1972 until 1974 he was a fellow at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland. He returned to the University of Utrecht in 1976 as an assistant professor and became full professor of physics in 1977.
Gerardus 't Hooft showed for the first time how to renormalize the electroweak theory of Sheldon Glashow, Abdus Salam and Steven Weinberg in 1971, building upon the work of his Phd supervisor, Martinus Veltman, with whom he shared the Nobel prize in 1999.
Equations in quantum field theory often seem to give nonsensical infinite predictions. Renormalization is the complex procedure necessary to show that they in fact do not. Every field theory must go through this grindstone to qualify as a tenable physical theory.
Fundamental interactions are described by qauge theories. These theories have two important features. The first is that particles of matter interact by exchanging gauge particles or particles which mediate the interaction. For electromagnetic interaction which is long-ranged the mediating particles are photons, which are massless. In the electroweak theory, which is a unification of the electromagnetic and the weak interactions, there are mediating particles, other than the massless photons, which have mass because the weak interactions are short-ranged. These particles are called W and Z bosons.
The second feature is that gauge theories have gauge symmetry which mathematically means that the equations of the theory can be written in an infinite number ways, all equivalent to one another. For actual calculations one chooses some particular form. This is called gauge fixing or choosing the gauge. Some choices may have advantages over other, such as making calculations or making Lorentz invariance manifest.
What 't Hooft did in 1971, four years after the work of Salam and Weinberg, was to put it on firm mathematical ground by showing that the electroweak theory is indeed renormalizable. To do this he invented a choice of gauge in which the infinities which occur in the calculations could be absorbed in the redefinition of the parameters of the theory.