  At an elementary level, we present some non-perturbative aspects of
non-abelian gauge theories in four dimensional space-time. Some rigorous
results have been obtained in the framework of supersymmetric theories, and a
very rich physics related to the strong-coupling dynamics of gauge fields has
been discovered. Some of the new phenomena which appeared may play a
significant role in future phenomenological models. However, quantitative
computations in non-supersymmetric theories, like quantum chromodynamics, is
out of reach for the moment.
