The current rapid development in information technology is leading to a massive change from the traditionally used passive optical components towards more and more active components. These cannot be produced without a sound theoretical basis. Usually, the design synthesis of optical thin films is based on the established transfer matrix method (TMM) for calculating the linear spectral response function of the coating, which is available in several software packages. Besides the linear spectral response function, the nonlinear optical spectral response is only covered by recent developments. In most cases, the nonlinear properties are directly linked to the field distribution generated by the interference in the layer system. To take nonlinearities into account, one possibility is to explicitly incorporate them into a nonlinear TMM (NTMM). Another possibility is to implement the task into a maxwell solver (method: finite difference time domain (FDTD)). Both methods increase the numerical effort considerably, while the NTMM has advantages in computational speed and the FDTD can be more accurate.