The book addresses aspects of QCD which are related to its underlying structure as a field theory and to its mechanisms. Perturbative expansions do not work at large distances for QCD: the hadron spectrum, the confinement of colour, its deconfinement at high temperatures and the breaking of chiral symmetry all need nonperturbative methods of analysis. Sum rules, chiral perturbation theory and the formulation of QCD on a lattice are some of the tools used to test models, like the stochastic vacuum, the instanton liquid or the consideration of monopoles in the vacuum to produce dual superconduct.