Fluids in Earth's subsurface are of great societal interest. Petroleum, fracturing, and geothermal fluids are basic components of the energy system; magmatic fluids in volcanoes are associated with natural hazards; the fossilized remains of ancient volcanic intrusions provide insights into past tectonic environments; and liquid water in ice plays a critical role in the response of the cryosphere to a changing climate. Such fluids are commonly contained in fractures. Fractures are pervasive in geologic media, and fluid-filled fractures are the dominant fluid pathway in media with low intrinsic permeability. In the cryosphere, fluid-filled fractures occur as glacial crevasses as well as thin sheets of water at the bed of glaciers. In volcanoes, such fractures occur as magma-filled dikes and sills, while in geothermal and hydrocarbon reservoirs they provide either preexisting or stimulation-induced fracture space.