Accretion disk outbursts, and their subsequent decline, offer a unique
opportunity to constrain the physics of angular momentum transport in hot
accretion disks. Recent work has centered on the claim by Cannizzo et al. that
the exponential decay of luminosity following an outburst in black hole
accretion disk systems is only consistent with a particular form for the
dimensionless viscosity, $\alpha=35(c_s/r\Omega)^{3/2}$. This result can be
understood in terms of a simple model of the evolution of cooling fronts in
accretion disks. In particular, the cooling front speed during decline is $\sim
\alpha_F c_{s,F}(c_{s,F}/r\Omega)^{n}$, where $F$ denotes the position of the
cooling front, and the exact value of $n$ depends on the hot state opacity,
(although generally $n\approx 1/2$). Setting this speed proportional to $r$
constrains the functional form of $\alpha$ in the hot phase of the disk, which
sets it apart from previous arguments based on the relative durations of
outburst and quiescence. However, it remains uncertain how well we know the
exponent $n$. In addition, more work is needed to clarify the role of
irradiation in these systems and its effect on the cooling front evolution.