  Nuclear magnetic resonance has emerged as a vital technique for investigating
strongly correlated electron systems, and is particularly important for
studying superconductivity. In this paper the basic features of NMR as a
technique for probing the superconducting state are reviewed. Topics include
include spin relaxation processes, studies of vortex lattices, and phenomena
associated with unconventional pairing symmetries. Recent experimental work is
reviewed, with a particular emphasis on the heavy fermion superconductors.
