Theoretical studies of nonlinear optical effects in nematic liquid crystals including degenerate four-wave mixing are presented. The optically induced Freedericksz transition which is essential for these effects is also described. Experimental investigations are performed using a homeotropically aligned MBBA thin film. Good agreement is obtained between the theoretical predictions and the experiments. Some potential applications of phase conjugation obtained by the backward degenerate four wave-mixing process in the field of adaptive optics and image processing are demonstrated. / Ph. D.