It has been shown that an approximate solution for the diffraction of an electromagnetic wave by an aperture in a plane conducting screen can be obtained from a single component of the Hertz vector and that the results can be applied to calculating the field of apertures which are sufficiently large as compared to the wavelength of the incident radiation. The solution rests upon an evaluation of an inhomogeneous scalar boundary‐value problem. An infinitesimal wavelength approximation leads to a simple Kirchhoff‐like formula which is found to be identical to that obtained by Neugebauer who derived it from geometrical optics as first approximation. The special case of diffraction by a circular aperture is treated in detail, and experimental measurements by which the theory was checked are cited.