  The central engines of Active Galactic Nuclei (AGNs) are powered by accreting
supermassive black holes, and while AGNs are known to play an important role in
galaxy evolution, the key physical processes occur on scales that are too small
to be resolved spatially (aside from a few exceptional cases). Reverberation
mapping is a powerful technique that overcomes this limitation by using echoes
of light to determine the geometry and kinematics of the central regions.
Variable ionizing radiation from close to the black hole drives correlated
variability in surrounding gas/dust, but with a time delay due to the light
travel time between the regions, allowing reverberation mapping to effectively
replace spatial resolution with time resolution. Reverberation mapping is used
to measure black hole masses and to probe the innermost X-ray emitting region,
the UV/optical accretion disk, the broad emission line region and the dusty
torus. In this article we provide an overview of the technique and its varied
applications.
