The following links provide a basic review of electromagnetic theory and the various numerical techniques used by EM.Cube's simulation engines:
Electrostatic & Magnetostatic Field Analysis
The Far-Field Approximation for Radiation & Scattering Problems
Basic Principles of The Finite Difference Time Domain Method
Basic Principles of SBR Ray Tracing
Basic Principles of The Method of Moments
Basic Principles of Physical Optics