In this thesis a new electromagnetic scattering problem is solved analytically. The structure considered is a parallel-plate waveguide in which a slotted wall is inserted. Exact expressions of the electromagnetic field and surface current densities in the whole structure, for the characteristic modes of the parallel-plate waveguide (TEM mode,TE and TM modes) are theoretically evaluated. Numerical calculations of the surface current densities on the metallic posts and parallel plates are performed too, with the assistance of Fortran 90 subroutines.