The most non-trivial of the established microscopic theories of physics is quantum chromodynamics, QCD, the theory of the strong interaction.A critical link between theory and experiment is provided by the methods of perturbative QCD, notably the well-known factorization theorems.Giving an accurate account of the concepts, theorems and their justification, this book is a systematic treatment of perturbative QCD.As well as giving a mathematical treatment, the book relates the concepts to experimental data, giving strong motivations for the methods.It also examines in detail transverse-momentum-dependent parton densities, an increasingly important subject not normally treated in other books.Ideal for graduate students starting their work in high-energy physics, it will also interest experienced researchers wanting a clear account of the subject.First published in 2011, this title has been reissued as an Open Access publication on Cambridge Core.