Parallel optical information processing systems are well suited for information reduction and preprocessing. Three important specifications for such applications have to be fulfiled: High space-bandwidth-product, amplification for signal restoring, and compensation of aberrations. The introduction of phase-conjugating elements into optical information processing systems will help to realize these specifications. The advantages of phase conjugation in optics, like exact counterpropagation and phase reconstruction, are well known, but the number of experimental realized applications is still small. This report gives a small insight into realised applications of phase conjugation. Setups of phase conjugating mirrors in combination with different types of interferometers (Michelson, Sagnac, and Fabry-Perot) will be presented. We report on applications on image subtraction, contrast amplification, phase visualisation, and parallel optical feedback systems. In all our experiments we used BaTiO3 photorefractive crystals and Ar + -ion laser for pump/signal waves.