Data from space missions of various kinds coupled with those from Earth-based observatories have allowed us to pass beyond the separate study of individual planets to the study of the Solar System as a composite physical system. We can identify those physical principles which are characteristic of planetary bodies over the full range of possible sizes. This article offers a broad general survey of the physics of planetary interiors in these terms, without concentrating in detail on any particular planetary body. The aim is to highlight significant areas which could repay future study, as well as to explain what is already understood. Magnetism will not be included nor will matters involving possible origins of the Solar System.