Rational homotopy theory is a subfield of algebraic topology. It has been an active area for more than 30 years, and no attempt has been made at writing a textbook in more than 15 years. Both notation and techniques of rational homotopy theory have been considerably simplified over the past 15 years, so that the older books in are already out of date. All three authors are considered to be among the leading experts in rational homotopy theory.