A comprehensive analysis of the theory of z-scan method has been reported here in detail. The experimental techniques and its recent applications on the determinations of optical nonlinearities of some organic materials such as tetrabenzporphyrin (TBP), phthalocyanine, bisbenzethiozole-substituted thiophene (BBTDOT) and didecyloxy substituted polyphenyl (DDOS) have also been described briefly.