Abstract This chapter reviews the use of oxygen-isotope ratios (i.e., 18 O/ 16 O) from fossil foraminifera in paleoclimate studies. 1 It focuses on the constraints given by oxygen-isotope measurements for the timing, structure, and mechanisms of glaciation. The literature on stable isotopes in foraminifera has expanded considerably since the review of early work by Duplessy (1978). The emphasis here is on new developments and what they say about oxygen-isotope measurements as an ice-volume proxy. The last deglaciation is compared with long-term patterns of Quaternary glaciation. The evidence is examined for the structure of the last deglaciation recorded by marine δ 18 O, its timing as determined by radiocarbon dating, and the constraints this gives for interpreting mechanisms of glacial/interglacial climate change.