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Reverberation mapping is a method of estimating the mass of an active galactic nucleus's (AGN's) supermassive black hole (SMBH) using the velocities suggested by transients, particularly the manner in which they occasionally grow brighter. It depends on the fact that AGNs often do show such transients. A pattern is the AGN grows brighter, then after some time, some of the AGN's lines broaden. The method assumes the brightening occurred at some portion of the material surrounding the black hole, and the observed subsequent line broadening was a response that occurred when the increased light reached some gas or plasma also surrounding the black hole, but near a different portion of the event horizon's surface. The the longer the delay, the further apart these two regions must be and the larger the black hole is assumed to be, and given the speed of light, the delay offers some information about this distance, giving a rough idea of the black hole's size. Such an increase in line broadening is referred to as a reverberation. Observing a number of a black hole's such transients would show a distribution of delays, from which conclusions can be drawn about the black hole's likely size, and from that, the mass.