Molecular assemblies prepared by the Langmuir-Blodgett technique provide useful structures to probe structure-property relationships for nonlinear optical processes. In this paper we present selected results of a comprehensive research program in which we study the Langmuir-Blodgett films of optically nonlinear organic structures. The films are carefully characterized by a variety of surface and spectroscopic techniques. Both second and third order processes have been investigated. A detailed study of third-order resonant nonlinearity in a Si-phthalocyanine using femtosecond degenerate four wave mixing is presented. Using a new approach of simultaneously monitoring signals corresponding to first and second order diffractions and their power dependence, important information on excitation dynamics is derived. The theoretical analysis shows that both first order and bimolecular exciton-exciton annihilation processes in the studied Si-phthalocyanine contribute to the decay of resonant third-order optical nonlinearity.