Two issues relevant to the nonlinear optical properties of nipi-type superlattices are discussed. First, an experimental comparison of photoinduced changes in the absorption coefficients of nipi, hetero-nipi, and heterostructure superlattices is presented. This shows the characteristic differences between those structures that make use of the modulation of internal electric fields and those that operate solely on the basis of free carrier effects. It is shown that the modulation of internal electric fields gives rise to large optical nonlinearities at low optical power levels. Second, a model that describes the lateral distribution of photoinjected charge carriers in an inhomogeneously excited nipi-type superlattice is discussed. It is shown that lateral drift dominates over diffusion, and that the characteristic drift length is large.