  We review our current understanding of the interior structure and thermal
evolution of Saturn, with a focus on recent results in the Cassini era. There
has been important progress in understanding physical inputs, including
equations of state of planetary materials and their mixtures, physical
parameters like the gravity field and rotation rate, and constraints on
Saturnian free oscillations. At the same time, new methods of calculation,
including work on the gravity field of rotating fluid bodies, and the role of
interior composition gradients, should help to better constrain the state of
Saturn's interior, now and earlier in its history. However, a better
appreciation of modeling uncertainties and degeneracies, along with a greater
exploration of modeling phase space, still leave great uncertainties in our
understanding of Saturn's interior. Further analysis of Cassini data sets, as
well as precise gravity field measurements from the Cassini Grand Finale
orbits, will further revolutionize our understanding of Saturn's interior over
the next few years.
