This chapter provides up-to-date coverage on the geophysical, chemical, and biological properties of the lakes that lie beneath the Antarctic ice cap (subglacial lakes). There are at least 150 lakes beneath this ice cap and many may be connected by networks of subglacial streams and rivers. The most well known of these lakes is Lake Vostok. Recent evidence indicates that subglacial lakes may initiate and maintain rapid ice flow, and should be considered in ice sheet mass balance assessments. The discovery of viable organisms in subglacial environments demonstrates that life has radiated into all aquatic habitats on the planet. Sub-glacial liquid environments offer an exciting frontier. Their study will provide an improved understanding of the coupling of geological, glaciological, and biological processes in the polar zones.