  The dual approach to the Ginzburg-Landau theory of a
Bardeen-Cooper-Schrieffer superconductor is reviewed. The dual theory describes
a grand canonical ensemble of fluctuating closed magnetic vortices, of
arbitrary length and shape, which interact with a massive vector field
representing the local magnetic induction. When the critical temperature is
approached from below, the magnetic vortices proliferate. This is signaled by
the disorder field, which describes the loop gas, developing a non-zero
expectation value in the normal conducting phase. It thereby breaks a {\it
global} U(1) symmetry. The ensuing Goldstone field is the magnetic scalar
potential. The superconducting-to-normal phase transition is studied by
applying renormalization group theory to the dual formulation. In the regime of
a second-order transition, the critical exponents are given by those of a
superfluid with a reversed temperature axis.
