Earthquakes have been clearly linked to subsurface fluid injection in wells at two places--near Denver at the Rocky Mountain Arsenal waste-disposal well and at the Rangely oil field in northwestern Colorado. The theory linking fluid pressure to earthquakes is based on the effective stress concept--i.e., that increases of pore pressure reduce the effective normal stress across fracture surfaces. At both the Rocky Mountain Arsenal and Rangely, evidence exists for substantial tectonic shearing stresses in the reservoir rock, although stress was below the critical value necessary to cause failure. At Denver, fluid injection relieved a fraction of the frictional resistance to shear fracture and earthquakes resulted. There is presently no way to determine before drilli g whether injection at a given site will produce earthquakes. At Denver and Rangely, the earthquakes appear to have originated entirely along preexisting faults. In placement of injection wells, existing faults should be avoided. Seismic surveillance during injection can provide early warning of inadvertently triggered earthquakes, and palliative measures then can be taken. At Rangely, the earthquakes have been drastically reduced in frequency by reducing pore pressures in the hypocentral region. On the basis of this experience, it appears that seismic activity due to waterflooding in oil fields can be controlled without seriously disrupting production of oil.