ABSTRACT
This chapter introduces theories which give a coherent description of all three elementary particle interactions. These theories are known as ‘Grand Unification Theories’ (GUT). Phenomenologically, the interactions which determine the elementary particle domain, namely the electromagnetic, the weak and the strong interaction have very different natures. The philosophy followed by the GUT calls for a representation of all elementary interactions based on gauge invariance with respect to a single simple group. The chapter discusses the characteristic features of quantum chromodynamics, the gauge theory of the strong interaction. It describes a class of models in which an attempt is made to approach a solution of a number of problems which are not considered in the normal GUT models. These are the so-called supersymmetric GUT models. The principal objectives are: to include gravitation into the unification and to explain the mass spectrum of the elementary particles.
