  QCD in the $\epsilon$-regime at nonzero baryon chemical potential $\mu$ is
reviewed. The focus is on aspects of the sign problem which are relevant for
lattice QCD. It is discussed how spontaneous chiral symmetry breaking and the
sign problem are related through the spectrum of the Dirac operator. The
strength of the sign problem is linked to the quark mass and the chemical
potential. Specific implications for lattice QCD are discussed.
