They are out there waiting for us, in the shadows, moving through the chilled void as they have for eons (Fig. 1.1). Hidden from Earthbound eyes by darkness and distance, these worlds are only now yielding their secrets to us. From humble moons to dwarf planets, the ice worlds display a wondrous symphony of geology, texture, and color. Canyons score the faces of worlds such as Ganymede, Enceladus, and Charon. Ridges raise a repeating tempo across the landscapes of Europa, Titan, and Pluto, and great stretches of sand dunes sigh across Titan and Pluto as well. Fanfares of cryogenic volcanoes rumble from Europa, Enceladus, and perhaps Ceres, Titan, Pluto, Ariel, and other small worlds. Some moons and dwarf planets may host oceans to rival the deepest seas on Earth. Internal forces crescendo as mountains thrust into alien skies. Dramatic gorges lay down a baseline deeper than anything seen on our own world. Strange concoctions of salts, methane, and ammonia play harmonies of umbers, ochers and yellows across the ices, colored by a descant of radiation from the Sun and nearby planets. The outer Solar System, once thought to harbor only dead globes of ice, has turned out to be an exhilarating and astonishing cacophony of sound, grace, and fury.