The interior structures of planets and satellites have been investigated through various methods. Density is the most fundamental information for estimating the composition of solar system bodies. Moment of inertia expressing the mass concentration is a key for investigating the interior structure. It can be obtained from spacecraft flyby with a planet or a satellite. Interior density structure of a planet is evaluated from gravity field which is estimated from orbital tracking of spacecraft around the planet. KAGUYA first successfully obtained the accurate far-side gravity field of the Moon. Detailed interior structure can be estimated from seismic methods. Direct seismic measurements had been developed on the nearside of the Moon by the Apollo project. Seismograms of moonquakes, reflecting less interior dissipation, are different from those of the Earth. Radar sounders have been successful in measuring subsurface structure of the Moon and Mars. The state of the planetary interior, especially of the core, can be discussed using the measurement of variation of planetary rotation. The study of the planetary interior is now being extended into extrasolar planets.