Under certain circumstances, the increased pore pressure resulting from fluid injection, whether for waste disposal, secondary recovery, geothermal energy, or solution mining, can trigger earthquakes. This report discusses known cases of injection-induced seismicity and how and why earthquakes may be triggered, as well as conditions under which the triggering is most likely to occur. Criteria are established to assist in regulating well operations so as to minimize the seismic hazard associated with deep well fluid injection.