  We give a pedagogical introduction to the concept that light quarks diffuse
in the QCD vacuum following the spontaneous breaking of chiral symmetry. By
analogy with disordered electrons in metals, we show that the diffusion
constant for light quarks in QCD is $D=2F_{\pi}^2/|\la\bar{q}q\to|$ which is
about 0.22 fm. We comment on the correspondence between the diffusive phase and
the chiral phase as described by chiral perturbation theory, as well as the
cross-over to the ergodic phase as described by random matrix theory. The
cross-over is identified with the Thouless energy $E_c=D/\sqrt{V_4}$ which is
the inverse diffusion time in an Euclidean four-volume $V_4$.
