Jupiter and Saturn are mostly made up of hydrogen and helium, but details of their precise composition and the evolution and structure of their interiors are hard to come by. In his Perspective, Fortney highlights recent model studies that have shed light on the likely structure and evolution of these giant planets. The models highlight the need for accurate computer simulations and laboratory data on hydrogen under high pressure. The Cassini mission, currently at Saturn, will also provide much-needed data to constrain the models.