Oilfield subsidence, caused by reservoir compaction, has occured worldwide from the North Sea to South America and elsewhere.  Subsidence is the movement of surface strata in response to a loss of underground support often associated with reservoir compaction from hydrocarbon withdrawal.  Why do compaction and subsidence occur?  The answer varies from the simple response of strata to a pore-pressure decline to a complex interaction of pressure, temperature, and fluid saturations all influenced by underburden reservoir, and overburden geometry and properties.