The first suggestion for the use of oxygen isotopes in cosmochemistry was that of H. C. Urey and colleagues in 1934, but appropriate instrumentation had not yet been developed. The modern era of oxygen isotope cosmochemistry began with the study of Apollo lunar samples in 1969 and of Allende refractory inclusions in 1973. The large (>5%) variations in 17O/16O and 18O/16O ratios, and small variations in 17O/18O were first interpreted as nucleosynthetic effects, but are now recognized to be the result of chemical processes early in Solar System history. Thus oxygen isotopes provide natural tracers for processes of formation of solid bodies in the inner Solar System. In particular, oxygen isotopes are very useful in recognizing genetic associations among meteorite groups. They also have been valuable in the study of parent body processes, such as metamorphism and aqueous alteration. There is conjecture that the ultimate cause of the oxygen isotope effects may be isotope-selective photodissociation of CO, which will be tested by isotopic measurement of solar oxygen and nitrogen collected in the NASA Genesis mission.