  It is well known that the outer parts of QSO accretion disks are prone to
selfgravity if heated solely by orbital dissipation. Such disks might be
expected to form stars rather than accrete onto the black hole. The arguments
leading to this conclusion are reviewed. Conversion of a part of the gas into
high-mass stars or stellar-mass black holes, and the release of energy in these
objects by fusion or accretion, may help to stabilize the remaining gas. If the
disk extends beyond a parsec, however, more energy is probably required for
stability than is available by turning half the gas into high-mass stars. Small
black holes are perhaps marginally viable energy sources, with important
implications (not pursued here) for the QSO spectral energy distribution, the
metallicity of the gas, microlensing of QSO disks, and perhaps
gravitational-wave searches. Other possible palliatives for selfgravity include
accretion driven by nonviscous torques that allow near-sonic accretion speeds
and hence lower surface densities for a given mass accretion rate. All such
modes of accretion face major theoretical difficulties, and in any case merely
postpone selfgravity. Alternatively, thin disks may not exist beyond a thousand
Schwarzshild radii or so (0.01 parsec), in which case QSOs must be fueled by
gas with small specific angular momentum.
