  Reverberation mapping is now a well-established technique for investigating
spatially-unresolved structures in the nuclei of distant galaxies with
actively-accreting supermassive black holes. Structural parameters for the
broad emission-line region, with angular sizes of microarcseconds, can be
constrained through the substitution of time resolution for spatial resolution.
Many reverberation experiments over the last 30 years have led to a practical
understanding of the requirements necessary for a successful program. With
reverberation measurements now in hand for 60 active galaxies, and more on the
horizon, we are able to directly constrain black hole masses, derive scaling
relationships that allow large numbers of black hole mass estimates throughout
the observable Universe, and begin investigating the detailed geometry and
kinematics of the broad line region. Reverberation mapping is therefore one of
the few techniques available that will allow a deeper understanding of the
physical mechanisms involved in AGN feeding and feedback at very small scales,
as well as constraints on the growth and evolution of black holes across cosmic
time. In this contribution, I will briefly review the background,
implementation, and major results derived from this high angular resolution
technique.
