The results of the study on phase conjugation of weak optical signals with an energy of several photons are considered. Basic concepts of a semi-classical theoretical approach to describe amplification and phase conjugation (PC) of such signals are presented. A review is given of the experimental investigations that allowed reaching a limiting (about 1 photon per mode) sensitivity of optical systems with PC-mirrors for a large $(\sim10^5)$ number of resolution elements. High-sensitivity PC-mirror optical systems are demonstrated for a variety of applications.