This is a collection of pedagogically written articles on quantum chromodynamics. Based on lectures given over the past years to graduate students, the articles are ideally suited as an introduction into modern research. The book contains concepts and applications of lattice gauge theories, finite temperature field theory, an article on the method of sum rules in QCD, and so on. Furthermore axial anomalies, the Skyrme model and recent developments of supersymmetric QCD are discussed in depth. A second volume on phenomenology is in preparation.