  I review the presence of the sign problem in lattice QCD at nonzero baryon
density and its relation with the overlap and Silver Blaze problems. I then
discuss progress in some cases where the sign problem can be handled, either
because the sign problem is absent or because it is milder than in full QCD.
Some time is spent on effective three-dimensional models, which can be treated
with a variety of methods. I conclude with a discussion of the applicability of
complex Langevin dynamics at nonzero density.
