The Galileo probe measurements of noble gases and major elements in the atmosphere of Jupiter provide a key constraint on the origin of the solar system's largest planet…provided we can determine the oxygen abundance! I will explain how. Likewise, at Saturn the Huygens probe will make key elemental measurements of Titan's atmosphere, which together with the known isotopic abundances will tell us how large satellites form around giant planets…with the help of modeling efforts that I will detail.