A theory of two-photon absorption in GaAs/GaAlAs superlattices is given in a zero and a quantizing magnetic field. Radiation is assumed to be polarized in the plane of the layer. Numerical results for the absorption coefficient are given for two superlattice periods. The threshold energy for two-photon absorption is found to be smaller for superlattices of smaller period, both in zero and non-zero magnetic fields. The selection rules for transition between Landau levels for left and right circular polarization are DELTAn = + 1 and - 1, respectively, in contrast to the selection rule DELTAn = 0 for one-photon absorption.