This text is a detailed version of material presented by both of us in seminars and lectures in the theory group of this department. Except for parts of Chapters 9 and 10 the material has also been covered in a series of seminars by one of us (M.-K.) in the Department of Physics of the University of Adelaide, Adelaide, Australia, in August and September 1985 and in the Department of Physics of Shanxi University, Taiyuan, China, in March and April 19 87. The interest and criticism of the audiences at these departments and, in particular, the support and enthusiasm of Professor A.W. Thomas (Adelaide) and Professor Zhang Jianzu (Taiyuan) are gratefully acknowledged.