ABSTRACT
This chapter introduces theories which give a coherent description of all several elementary particle interactions. Phenomenologically, the interactions which determine the elementary particle domain, namely the electromagnetic, the weak and the strong interaction have very different natures. The chapter discusses the characteristic features of quantum chromodynamics (QCD), the gauge theory of the strong interaction. It also discusses the basic concepts of grand unification theories. The chapter 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 Grand Unification Theories (GUT) models. The principal objectives are: to include gravitation into the unification; and to explain the mass spectrum of the elementary particles. Every GUT model based on normal gauge symmetry may be extended to a supersymmetric version. The symmetry between fermions and bosons means that every fermion has a bosonic partner in the same supermultiplet, and vice-versa.
