Hot Jupiter is a designation that is often assigned to gas giant planets in short-period orbits around their parent stars. The characteristics of Hot Jupiters are not well defined, but the orbital period must be short enough (say less than 10 or 20 days) for the planet to be truly “hot,” and the mass must be large enough (say more than about one third of a Jupiter mass) for the planet to be a gas giant. It is commonly accepted that Hot Jupiters originally form much farther from their host stars and migrate inwards either by interaction with the circumstellar disk from which they formed or by gravitational interactions with other planets in the system. For reasons that are not fully understood, there is a pileup in the number of Hot Jupiters with orbital periods near 3 days around solar-type stars. The first Hot Jupiter, which is also the first detected extrasolar planet around Sun-like stars, was found in 1995, orbiting the star 51 Peg (Mayor and Queloz 1995).