This thesis is primarily concerned with some laser-induced nonlinear reflection and refraction phenomena in lithium niobate (LiNbO₃) crystals. These are photorefraction and phase conjugation. These phenomena are caused by nonlinear optical processes and the underlying basic theory is presented. A critical review of the theory of phase conjugation by Degenerate Four-Wave Mixing (DFWM) of coherent laser beams is given and an analysis and a simple mathematical description of photorefraction, i.e. light-induced refractive index changes, are presented. Laser beam coupling by two-wave mixing and phase conjugation by four-wave mixing in photorefractive media are discussed in detail. Relevant physical properties of LiNbO₃ crystals were measured and are described. An account of the design of experiments and the apparatus used in the present study to investigate photorefraction and phase conjugation in LiNbO₃ are presented. The characteristics of the He-Cd laser used in the experimental investigation were measured and are described. Experiments were conducted using two-wave mixing to yield the photorefractive parameters of LiNbO₃ which were then used in the interpretation of the experiments in which phase conjugation was observed in DFWM studies. Details of the procedures and the results are given together with estimates obtained for diffusion and photovoltaic field strengths in the conditions used in the present investigation. Photographic evidence of the corrective removal of optical distortion from laser beam propagating through LiNbO₃ cyrstal is given.