Third-order nonlinear optical interactions have been studied for thin films of a number of organic polymeric systems to derive an understanding of these processes. Picosecond and subpicosecond degenerate four wave mixing was used to derive the nature of the nonlinearity and the associated response time. The dominant contribution is found to be derived from the 7-electron conjugation and a subpicosecond response is confirmed. In the case of a polymeric photoconductor, the nonlinearity due to 7-electrons dominates if the structure is conjugated. In the case of a non-conjugated photoconductor, contribution due to charge-carriers seems to dominate and has response times of hundreds of picoseconds. Vibrationally resonance enhanced nonlinearity is also investigated by coherent Raman scattering. The resonance enhancement is three orders of magnitude, and vibrational dephasing is found to be extremely fast. For studies of optical nonlinearity in monolayer and successively-built multilayer Langmuir-Blodgett films, the surface plasmon technique is found to be ideally suited. A power dependence study of the total reflectivity vs the internal ii Bence angle in a prism coupling geometry is used to obtain the magnitude as well as the sign of x (3).