Foraminifera commonly dominate ocean-floor eukaryotic communities. They also are the most abundant benthic organisms to be preserved in the post-Paleozoic deep-sea fossil record. Living faunas contain numerous delicate, soft-bodied agglutinated forms which have virtually no fossilization potential, whereas fossil faunas usually consist of calcareous and more robust agglutinated taxa. Recent observations have emphasized the important, often active role that foraminifera play in the dynamics and structuring of deep-sea benthic ecosystems. Foraminiferal abundances are closely linked to levels of organic matter input, and to dissolved oxygen concentrations in the near-bottom water which, in part, are related inversely to the magnitude of organic fluxes. Food and oxygen requirements also control the microhabitat preferences of species, which are reflected in their test morphology. Thus, the abundance of modern species, species assemblages, and test morphotypes can be related to regional organic carbon inputs and oxygen concentrations. Specific assemblages are often associated with particular kinds of input (for example, phytodetrital pulses) or with low-oxygen settings. These biological observations have applications in paleoceanography. Quaternary and Neogene faunas contain many extant species, preserving an environmental signal that can be interpreted by analogy with modern assemblages. In older sediments, which contain few extant species, morphotype analysis provides a useful approach. Autecological observations on living calcareous species also provide the basis for interpreting the stable carbon isotope record. Further research and collaborative interactions between biologists and geologists should help to improve understanding of the paleoceanographic record.