  We describe the current state of knowledge about Mercury's interior
structure. We review the available observational constraints, including mass,
size, density, gravity field, spin state, composition, and tidal response.
These data enable the construction of models that represent the distribution of
mass inside Mercury. In particular, we infer radial profiles of the pressure,
density, and gravity in the core, mantle, and crust. We also examine Mercury's
rotational dynamics and the influence of an inner core on the spin state and
the determination of the moment of inertia. Finally, we discuss the
wide-ranging implications of Mercury's internal structure on its thermal
evolution, surface geology, capture in a unique spin-orbit resonance, and
magnetic field generation.
