  The wealth of observational data about Jupiter and Saturn provides strong
constraints to guide our understanding of the formation of giant planets. The
size of the core and the total amount of heavy elements in the envelope have
been derived from internal structure studies by Saumon & Guillot (2004). The
atmospheric abundance of some volatile elements has been measured {\it in situ}
by the {\it Galileo} probe (Mahaffy et al. 2000, Wong et al. 2004) or by remote
sensing (Briggs & Sackett 1989, Kerola et al. 1997). In this Letter, we show
that, by extending the standard core accretion formation scenario of giant
planets by Pollack et al. (1996) to include migration and protoplanetary disk
evolution, it is possible to account for all of these constraints in a
self-consistent manner.
