Reverse-time migration is the process that transfers recorded scattered data into the subsurface image by means of the two-way wave equation. The wave equation can be made to run both forward and backward in time. When the wave equation runs forward in time, it is a modeling process that transfers the subsurface reflectivity into scattered fields. When the wave equation runs backward in time, it is a migration imaging process that transfers the recorded scattered wavefields into the subsurface reflectivity image. The two processes are conjugate to each other (Claerbout, 1991), because the wave propagation wavefront is reversible. Thus the input to reverse-time migration is the recorded scattered data without divergence correction.