Integral equations for both interior and exterior fields of microstrip antennas are formulated in terms of the tangential electric and magnetic field distributions over the conducting patch and dielectric substrate surfaces. Boundary conditions are applied, and a set of integral equations for the above surface distributions are obtained. Using a moment method, these integral equations are then reduced to a matrix equation, the solution of which gives the required field distributions. The method is then applied to both square and rectangular path geometries and their near- and far-field distributions are determined. The far-field results are compared with the experimental data and with the results of approximate analytic methods. The usefulness of the method for solving, numerically, the problem of microstrip radiators is then discussed.