We discuss the interior structure and composition of giant planets, and how this structure changes as these planets cool and contract over time. Here we define giant planets as those that have an observable hydrogen-helium envelope, which includes Jupiter-like planets, which are predominantly H/He gas, and Neptune-like planets which are predominantly composed of elements heavier than H/He. We describe the equations of state of planetary materials and the construction of static structural models and thermal evolution models. We apply these models to transiting planets close to their parent stars, as well as directly imaged planets far from their parent stars. Mechanisms that have been postulated to inflate the radii of close-in transiting planets are discussed. We also review knowledge gained from the study of the solar system’s giant planets. The frontiers of giant planet physics are discussed with an eye towards future planetary discoveries.