A comprehensive report on the largest planet (and "the largest true planet possible"), including considerable background on the history, methods and findings of solar system astronomy and copious comparative tables on the size, orbits, position, etc. of all the planets. Asimov explains Jupiter's unusual brightness and shape, considers the planet's satellites and their part in discoveries about Jupiter, moons in general, and even of the speed of light. Jupiter's gravitational influence on the sun and on other planets and its effect on the asteroids and comets is surveyed and the author ends with views of Jupiter and its atmosphere as seen by a possible space traveler of the future, then an exploration of puzzles still to be solved -- some perhaps by the Pioneer 10 probe slated to fly by Jupiter on December 3 of this year. There is much raw data here for student reports, and for more serious readers a solid astronomical context as well.