  'Empirical' models (pressure vs. density) of Uranus and Neptune interiors
constrained by the gravitational coefficients J_2, J_4, the planetary radii and
masses, and Voyager solid-body rotation periods are presented. The empirical
pressure-density profiles are then interpreted in terms of physical equations
of state of hydrogen, helium, ice (H_2O), and rock (SiO_2) to test the physical
plausibility of the models. The compositions of Uranus and Neptune are found to
be similar with somewhat different distributions of the high-Z material. The
big difference between the two planets is that Neptune requires a non-solar
envelope while Uranus is best matched with a solar composition envelope. Our
analysis suggests that the heavier elements in both Uranus' and Neptune's
interior might increase gradually towards the planetary centers. Indeed it is
possible to fit the gravitational moments without sharp compositional
transitions.
