  These lectures present an elementary introduction to quantum gauge fields.
The first aim is to show how, in the tree approximation, gauge invariance
follows from covariance and unitarity. This leads to the standard construction
of the Lagrangian by means of covariant derivatives in a form that unifies the
massive and the massless case. Having so identified the classical theory, the
Faddeev-Popov quantization method is introduced and the BRS invariance of the
resulting action is discussed.
