This edition of the book has been extended to take account ofone of these developments， one which was just hinted at in thesecond edition. A close and very fruitful relationship has beendiscovered between geometric invariant theory for quasi projectivecomplex varieties and the moment map in Symplectic geometry， and achapter has been added describing this relationship and some of itsapplications. In an infinite-dimensional setting the moment maplinks geometric invariant theory and Yang-Mills theory， which hasof course been the focus of much attention among mathematiciansover the last fifteen years.
　　In style this extra chapter is closer to the appendices added inthe second edition than to the original text. In particular noproofs are given where satisfactory references exist.