The paper discusses basic principles and validity limits of the field-to-transmission line coupling calculating using Transmission Line (TL) theory. Attention is given to the EMP-power lines interaction. A review of the fundamental equations and method of solution is given, considering that two equivalent formulations are possible. These result in two equivalent circuits for an elementary section of the transmission line, each characterized by different excitation source terms. The excitation sources for the case of a nuclear EMP excitation are modeled, and the solution of the equations is given taking into account the effect of a lossy earth serving as a return conductor. In order to assess the validity limits of the TL approximation, a comparison is made between results obtained by the authors using this approach, with those published in the literature using the scattering theory. The effect of shield overhead wires on the response of a power transmission line to an EMP is investigated. It is shown that overhead shield wires, placed in the vicinity of phase conductors for the purpose of providing protection of the system from lightning surges, can be considered effective also when the power line is illuminated by an incident electromagnetic pulse.