Background seismicity refers to regional seismicity that cannot be associated with known regional faults. The formulation of background seismicity is an integral part of all regional seismicity models. In most regions, the seismicity on crustal faults and subduction zones cannot fully capture the recorded regional spatial distribution of small and intermediate-magnitude earthquakes. Background seismicity models are designed to capture the spatial distribution of shallow and intermediate depth seismicity, given the recorded data on past earthquakes and the expected randomness in locations and magnitudes of the potential earthquakes.