Microscopic calculations are reported of linear and quadratic nonlinear optical response in Langmuir-Blodgett films. The films are modeled as ordered layers of molecules that can tilt from the vertical and have a higher polarizability and first hyperpolarizability in the head than in the tail. The local electric field varies little after the first layer and negligibly after the second, which simplifies the analysis. Tilted two-layer Y-type films can have quadratic susceptibility components as large as X- or Z-type ones. Overall the results provide a systematic account of the linear and nonlinear optical properties as a function of molecular and film structure that should assist interpretation of experimental results. (C) 2000 American Institute of Physics. [S0021-9606(00)70115-2].