  We express the heavy quark diffusion coefficient as the temporal variation of
a Wilson line along the Schwinger-Keldysh contour. This generalizes the
classical formula for diffusion as a force-force correlator to a non-abelian
theory. We use this formula to compute the diffusion coefficient in strongly
coupled $\N=4$ Yang-Mills by studying the fluctuations of a string in
$AdS_5\times S_5$. The string solution spans the full Kruskal plane and gives
access to contour correlations. The diffusion coefficient is
$D=2/\sqrt{\lambda} \pi T$ and is therefore parametrically smaller than
momentum diffusion, $\eta/(e+p)=1/4\pi T$. The quark mass must be much greater
than $T\sqrt{\lambda}$ in order to treat the quark as a heavy quasi-particle.
The result is discussed in the context of the RHIC experiments.
