  We consider observational and theoretical estimates of the accretion disc
viscosity parameter $\alpha$. We find that in thin, fully-ionized discs, the
best observational evidence suggests a typical range $\alpha \sim 0.1 - 0.4$,
whereas the relevant numerical simulations tend to derive estimates for
$\alpha$ which are an order of magnitude smaller. We discuss possible reasons
for this apparent discrepancy.
