  We present a review of Saturn's interior structure and thermal evolution,
with a particular focus on work in the past 5 years. Data from the Cassini
mission, including a precise determination of the gravity field from the Grand
Finale orbits, and the still ongoing identification of ring wave features in
Saturn's C-ring tied to seismic modes in the planet, have led to dramatic
advances in our understanding of Saturn's structure. Models that match the
gravity field suggest that differential rotation, as seen in the visible
atmosphere, extends down to at least a depth of 10,000 km (1/6$^{\rm th}$ the
planet's radius). At greater depths, a variety of different investigations all
now point to a deep Saturn rotation rate of 10 hours and 33 minutes. There is
very compelling evidence for a central heavy element concentration (``core''),
that in most recent models is 12-20 Earth masses. Ring seismology strongly
suggests that the core is not entirely compact, but is dilute (mixed in with
the overlying H/He), and has a substantial radial extent, perhaps out to around
one-half of the planet's radius. A wide range of thermal evolution scenarios
can match the planet's current luminosity, with progress on better quantifying
the helium rain scenario hampered by Saturn's poorly known atmospheric helium
abundance. We discuss the relevance of magnetic field data on understanding the
planet's current interior structure. We point towards additional future work
that combines seismology and gravity within a framework that includes
differential rotation, and the utility of a Saturn entry probe.
