The real-time information processing and manipulation of electromagnetic waves using nonlinear Qptical techniques has resulted in a myriad of new applications in diverse fields such as quantum electronics, image processing, optical computing, adaptive optics, and nonlinear spectroscopy. In this paper, we review and explore the field, provide a historical perspective, analyze several of the nonlinear interactions useful for the generation of phase-conjugate replicas, and conclude with a brief survey of potential applications and suitable nonlinear media.