Primitive bodies (comets, Centaurs, trans-Neptunian objects, etc.) are the best-preserved remnants of the Solar Nebula. Their composition and structure harbor clues to the origin of solar and exosolar planets and the evolution of protoplanetary disks. The capabilities of the Gran Telescopio Canarias (GTC) and its instrumentation provide us a unique opportunity to accomplish an integrated study of the physical properties of these remote small bodies.