This note addresses two methods of computing the imaging condition for prestack migration of common-shot seismic data; our work is based on the ideas from reverse-time migration for both poststack (Loewenthal and Mufti, 1983; McMechan, 1983) and prestack data (Chang and McMechan, 1986). In reverse-time migration of poststack data, the whole stacked section is backward-extrapolated in time, with half of the medium velocity to time zero. All exploding reflectors are imaged at once at time zero. The time zero is referred to as the imaging condition. In prestack migration, the imaging condition is more involved. Each spatial grid point (treated as a point diffractor) has a different excitation time, which is equal to the one-way traveltime from the source to that grid point. Each point diffractor is imaged separately at its excitation (the "imaging time").