The diffraction of electromagnetic waves by circular apertures in extended conducting screens and by complementary obstacles was investigated experimentally to determine the structure of the field in the near zone. Techniques for measuring both the electric and magnetic field distributions are presented and the general experimental problems are discussed. Data are given for the tangential components of the electric and magnetic vectors in the principal planes in the immediate neighborhood of the diffracting structures, and axial distributions for the corresponding components. The results extend the range of the data available previously. The predicted uniformity of the tangential magnetic field in the aperture plane, for the case of a wave incident normally, is verified. An experimental test was made also of the electromagnetic Babinet principle.