From the reviews: "... They (Gabriel and Roiter) are pioneers in this subject and they have included proofs for statements which in their opinions are elementary, those which will help further understanding and those which are scarcely available elsewhere. They attempt to take us up to the point where we can find our way in the original literature. ..." The Mathematical Gazette, 1993 "... The standard of this text is high and will be definitely appreciated by the algebraic community." Monatshefte Mathematik, 1994 "..This book is very welcome because it presents some basic material and at the same time it presents some new insights of the theory. ..." Zentralblatt für Mathematik, 1996