  We show that the Keplerian thin disk in quiescent Soft X-ray Transients
cannot extend down to the last stable orbit around the central black hole. We
analyse the properties of the Narayan, McClintock & Yi (1996) model of
quiescent Soft X-ray Transients in which the cold Keplerian disk has its inner
edge at a large transition radius and transforms to a hot, advection-dominated
flow on the inside. We show that outbursts of transient sources could be
triggered in this model either by a pure thermal accretion disk instability or
by a disk instability generated by an enhanced mass transfer from the stellar
companion. Both mechanisms operate in the outer thin disk and could be at work,
either in different systems or in the same system at different epochs,
depending on the mass transfer rate and the value of the viscosity parameter
$\alpha_t$ of the thin disk. We show that the recurrence time between outbursts
in SXTs can be explained with values of $\alpha_t$ similar to these required by
the dwarf nova disk instability model instead of the unreasonably low values
needed in the model in which the thin disks extends down to the last stable
orbit. We extend the Narayan, McClintock & Yi (1996) model to the case when the
outer disk is non-stationary. We show that such disk is too cold to account for
the observed UV flux. This difficulty is common to all models in which the
outer disk is assumed to be optically thick.
