Trojan and Centaurs are primitinve, peculiar objects orbiting in the middle solar system. Both groups characteristically have low albedos and red colors. Physical observations of Trojans reveal featureless reddish spectra, implying surfaces probably rich in complex solid materials. The interiors are expected to be rich in H2O ice and other volatile material. Centaurs have surfaces showing dramtatically different spectral reflectances, from neutral to very red. Some spectra are featureless, while others show signatures of water ice, methanol, or other light hydrocarbons. Trojans were formed near Jupiter's orbit, while Centaurs where formed far beyond Jupiter's orbit, but both were formed at low temperatures at which water exists as solid ice.