Analytical and numerical the Beam Propagation Method techniques were used to analyze a variety of nonlinear optics phenomena at surfaces and in thin film waveguides. A number of new device possibilities for all optical signal processing were identified. New nonlinear guided-wave phenomena have been predicted, of which many are of interest for potential experimental and device applications. The highlights are 1 The analytical solutions for highly nonlinear waves guided by single interfaces and thin films extended to realistic media which exhibit saturation in the optically induced index change. 2 Beam propagation techniques used to find the stability of nonlinear guided waves for the first time. Furthermore, non-stationary nonlinear guided waves were found, also for the first time. 3 The generation of solitons predicted for nonlinear waveguides excited at high powers. Applications to optical limiters and optical logic via the exchange of solitons identified. The effects of non-ideal excitation conditions, waveguide absorption, and index saturation investigated. 4 Analytical and numerical solutions for nonlinear TM-polarized guided waves obtained for the first time. 5 The feasibility of light-by-light modulation using nonlinear guided waves of both polarizations predicted. 6 The origin of bistability in nonlinear prism couplers fully explained for the first time. 7 The response of nonlinear directional couplers to very high excitation calculated, resulting in unexpected steady-state conditions.