This book provides an introduction to Quantum Chromodynamics (QCD), the theory of strong interactions. It covers in full detail both the theoretical foundations and the experimental tests of the theory. Although the experimental chapters focus on recent measurements, the subject is placed into historical perspective by also summarizing the steps which lead to the formulation of QCD. Measurements are discussed as they were performed by the LEP experiments at CERN, or at hadron-hadron and lepton-hadron colliders such as the TEVATRON at Fermilab and HERA at DESY. Emphasis is placed on high energy tests of QCD, such as measurements of the strong coupling constant, investigations of the non-abelian structure of the underlying gauge group, determinations of nucleon structure functions, and studies of the non-perturbative hadronization process. This excellent text gives a detailed overview of how QCD developed in the 20th century and where we stand with respect to a quantitative understanding after the turn of the millenium. The text is intended for graduate and postgraduate students as well as researchers, and includes numerous problems and solutions.