Correlation functions in the QCD vacuum

Correlation functions are one of the key tools used to study the structure of the QCD vacuum. They are constructed out of the fundamental fields and can be calculated using quantum-field-theory methods, such as lattice gauge theory. One can obtain many of these functions using the rich phenomenology of hadron physics. They are also the object of study in various quark models of hadronic structure. This review begins with available phenomenological information about the correlation functions, with their most important properties emphasized. These are then compared with predictions of various theoretical approaches, including lattice numerical simulations, the operator product expansion, and the interacting instanton approximation.