  The evolution of ideas which has led from the first proofs of the
renormalizability of non-abelian gauge theories, based on Slavnov--Taylor
identities, to the modern proof based on the BRS symmetry and the master
equation is recalled. This lecture has been delivered at the {\bf Symposium in
the Honour of Professor C. N. Yang}, Stony-Brook, May 21-22 1999.
