Microseismic events are very weak earthquakes that occur at very small spatial scales. Microseismic events can result from natural forces such as tectonic motions (natural seismicity) or they can be induced by man-made changes to the natural stress-strain conditions in the earth (induced seismicity). There is a growing awareness that fluid injection into or fluid withdrawal from the earth's subsurface (e.g., oil/gas production, geothermal energy production, waste-fluid disposal), which changes the pore pressure, stress, and strain conditions in the rock, can induce microseismic events and might eventually lead to larger “felt” seismic events. (See the June 2015 TLE special section on injection-induced seismicity.)