Benthic foraminifera, shell-bearing protists, are familiar from geological studies. Although many species are well known, undescribed single-chambered forms are common in the deep sea. Coastal and sublittoral species often have restricted distributions, but wide ranges are more frequent among deep-water species, particularly at abyssal depths. This probably reflects the transport of tiny propagules by currents across ocean basins that present few insurmountable barriers to dispersal, combined with slow rates of evolution. Undersampling of the vast deep-sea habitat, however, makes it very difficult to establish the ranges of less common foraminiferal species, and endemism may be more prevalent than currently realized. On continental slopes, some species have restricted distributions, but wide-ranging bathyal species that exhibit considerable morphological variation are more common. This may be linked to the greater heterogeneity of continental slopes compared with oceans basins. Improved knowledge of deep-sea foraminiferal biogeography requires sound morphology-based taxonomy combined with molecular genetic studies.