  We discuss technologies for micro-arcsec echo mapping of black hole accretion
flows in Active Galactic Nuclei (AGN). Echo mapping employs time delays,
Doppler shifts, and photoionisation physics to map the geometry, kinematics,
and physical conditions in the reprocessing region close to a compact
time-variable source of ionizing radiation. Time delay maps are derived from
detailed analysis of variations in lightcurves at different wavelengths. Echo
mapping is a maturing technology at a stage of development similar to that of
radio interferometry just before the VLA. The first important results are in,
confirming the basic assumptions of the method, measuring the sizes of AGN
emission line regions, delivering dozens of black hole masses, and showing the
promise of the technique. Resolution limits with existing AGN monitoring
datasets are typically ~5-10 light days. This should improve down to 1-2 light
days in the next-generation echo mapping experiments, using facilities like
Kronos and Robonet that are designed for and dedicated to sustained
spectroscopic monitoring. A light day is 0.4 micro-arcsec at redshift 0.1, thus
echo mapping probes regions 10^3 times smaller than VLBI, and 10^5 times
smaller than HST.
