This book is the sequel to Geometric Transfrmations I and II, volumes 8 and 21 in this series, but can be studied independently. It is devoted to the treatment of affine and projective transformations of the plane; these transformations include the congruences and similarities investigated in the previous volumes. The simple text and the many problems are designed mainly to show how the priniciples of affine and projective geometry may be used to furnish relatively simple solutions of large classes of problems in elementary geometry, including some straight edge construction problems. In the Supplement, the reader is introduced to hyperbolic geometry. The latter part of the book consists of detailed solutions of the problems posed throughout the text.