John Collins is Distinguished Professor of Physics at Pennsylvania State University. He has lengthy experience in perturbative QCD. He has proved a number of the fundamental theorems which form the main content of this book, and has a record of formulating and deriving novel results in QCD. During his career he has received several awards, including a Guggenheim fellowship, a Humboldt Research Award, a Mercator professorship and the J. J. Sakurai prize.