  We start by giving a brief introduction to string theory with emphasis on the
example of the bosonic string. In order to fully appreciate string theory it is
necessary to study the dynamics of the surface that the string traces out when
moving in space-time. This is described by what is known as conformal field
theory, which is the subject of chapter two. After this digression we return to
string theory to give some insight into how the results of the articles
contained in this thesis could provide with interesting ingredients for
constructions of realistic string theories. This is also continued on to some
extent in chapter seven and eight. The articles in this thesis are discussing
mainly two subjects, gauged WZNW models and affine branching functions. The
background for those issues is given in chapter seven and eight. In the
articles contained in this thesis we use two major tools. First affine Lie
algebras and its representation theory, and secondly BRST quantization. In
chapter four and five we deal with affine Lie algebras, and in chapter six we
try to illuminate some issues on BRST quantization.
