  In the core-accretion model, gas-giant planets form solid cores which then
accrete gaseous envelopes. Tidal interactions with disk gas cause a core to
undergo inward type-I migration in 10^4 to 10^5 years. Cores must form faster
than this to survive. Giant planets clear a gap in the disk and undergo inward
type-II migration in <10^6 years if observed disk accretion rates apply to the
disk as a whole. Type-II migration times exceed typical disk lifetimes if
viscous accretion occurs mainly in the surface layers of disks. Low turbulent
viscosities near the midplane may allow planetesimals to form by coagulation of
dust grains. The radius r of such planetesimals is unknown. If r<0.5 km, the
core formation time is shorter than the type-I migration timescale and cores
will survive. Migration is substantial in most cases, leading to a wide range
of planetary orbits, consistent with the observed variety of extrasolar
systems. When r is of order 100m and midplane alpha is of order 3 times 10^-5,
giant planets similar to those in the Solar System can form.
