The theory of fixed point classes (also known as Nielsen fixed point theory) examines the fixed points of an arbitary mapping of any connected finite polyhedron into itself. Its main problem is to determine a lower bound of the geometric countings (i.e. the actual numbers) of fixed points of all mappings homotopic to the given mapping. The existence problem of fixed points is evidently a particular case. The present book is an English translation of the revised second Chinese edition. It is a fundamental representation of the important theory, presented in a more elementary and geometric way than the existing literature. Thus, it will serve as a good introduction to the important field of algebraic topology and geometry.