  We review the relations between (twisted) supersymmetric gauge theories in
four dimensions and moduli problems in four-dimensional topology, and we study
in detail the non-abelian monopole equations from this point of view. The
relevance of exact results in N=1 and N=2 supersymmetric gauge theories to the
computation of topological invariants is emphasized. Some background material
is provided, including an introduction to Donaldson theory, the twisting
procedure and the Mathai-Quillen formalism.
