  Echo mapping exploits light travel time delays, revealed by multi-wavelength
variability studies, to map the geometry, kinematics, and physical conditions
of reprocessing sites in photo-ionized gas flows. In AGNs, the ultra-violet to
near infra-red light arises in part from reprocessing of EUV and X-ray light
from a compact and erratically variable source in the nucleus. The observed
time delays, 0.1-2 days for the continuum, 1-100 days for the broad emission
lines, probe regions only micro-arcseconds away from the nucleus. The continuum
time delays map the temperature-radius profiles of the AGN accretion discs. The
emission-line delays reveal radially stratified ionization zones, identify the
nature of the gas motions, and estimate the masses of the central black holes.
By using light travel time to measure the sizes of AGN accretion discs and
photo-ionized zones, echo mapping offers two independent ways to measure
redshift-independent distances to AGNs.
