This book provides an introduction to Quantum Chromodynamics (QCD), the theory of strong interactions. It places equal weight on the theoretical foundations and 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 colliders such as the Tevatron at Fermilab and Hera at investigations of the non-abelian structure of the underlying gauge group, determinations of nucleon structure functions, and studies of the non-perturbative hadronization process.