A review is given about the assumptions the development, and the results of the ripple theory. The limits of the linear theory are pointed out. The result of this theory shows a very strong coupling of the magnetization dispersion perpendicular to the mean magnetization, which is caused by the internal demagnetizing field. The exchange constant of this magnetostatic coupling exceeds the quantum mechanical exchange constant by orders of magnitude. The nonlinear ripple theory includes the component of the internal demagnetizing field parallel to the mean magnetization. This field becomes important when the rms values of the magnetization dispersion are . The nonlinear theory explains all deviations from a single domain behavior without introducing any special model.