The maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics. Although most recent investigations of mantle seismicity and dynamics state that seismicity extends to ''about 700 km,'' focal depths of 720 km have been reported for large earthquakes, as well as depths greater than 800 km for small events. We have examined focal depths of events in several catalogs and conclude that (1) the deepest reliable focal depths are at about 670-680 km, and events with focal depths beneath 600 km occur in several widely separated geographic regions, (2) observations supporting focal depths exceeding 680 km are usually few, poor, or inconsistent, (3) the reduction of seismic activity beneath about 650 km is quite abrupt. However, we cannot determine whether activity stops completely at 670--680 km, or the maximum size of events decreases gradually beneath 650 km, with magnitudes of 6.5-7.0 occasionally occurring at 650 km, magnitudes of 5.5-6.0 at 680 km, 4.0-4.5 at 695 km, etc. Several models can explain the absence of seismic activity in the lower mantle. Two models are consistent with the abrupt termination of activity at about 650-680 km in widely separated areas: a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle.