Methods involving two-beam interferometers which include one photorefr
active crystal as a component are reviewed. Such an element can perfor
m as a transient hologram, as a time-average hologram, as a phase conj
ugate mirror (for compensation of aberrations or self-reference interf
erometric systems), as a beam amplifier, or as an adaptive element. Sp
ecial attention to two-beam interferometers and linear propagation med
ia are given.