Some approaches for analyzing the high-frequency electromagnetic scattering by open-ended waveguide cavities with a planar interior termination are discussed. The contribution to scattering arising from the open end is found using the geometrical theory of diffraction based equivalent current method, while the interior cavity scattering contribution can be treated by several different approaches, for example, hybrid modal, ray, or beam methods; other contributions are ignored for the reasons outlined in the paper. Numerical results based on these methods are presented.