Physical Properties of Kuiper Belt Objects

The discovery of the first object in the Kuiper belt -- a formerly hypothetical ancient reservoir of icy objects located beyond Neptune's orbit -- started a revolution in our understanding of the outer Solar System; there was no longer a sharp edge to our planetary system at Pluto's orbit. About 1000 Kuiper belt objects (KBOs), intermediate in size between comets and planets, are now known to exist on orbits about the Sun. Since KBOs are the preserved building blocks of an outer Solar System planet, studying their physical and chemical properties provides an opportunity for a better understanding of the formation and evolution of the Solar System. Here we describe the progress made by ground-based and orbiting telescopes in studying the physical and chemical properties of KBOs over the last 15 years.