Coupling between two waveguide-fed planar slot arrays has been investigated using full-wave analysis. The analysis employs the method-of-moments solution to the pertinent coupled integral equa- tions for the aperture electric field of all slots. In order to compute coupling between two arrays, the input port of the first array is excited with a TE10 mode wave while the second one is match-termi- nated. After solving the moment method matrix equations, the aperture fields of all slots are obtained and thereby the TE10 mode wave received at the input port of the second array is determined. Coupling between two arrays is the ratio of the wave amplitude arriving in the second array port to the incident wave amplitude at the first array port. The coupling mechanism has been studied as a function of spacing between arrays in different directions, e.g. the electric field plane, the magnetic field plane, and the diagonal plane. Computed coupling values are presented for differ- ent array geometries.