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The progress of high power electromagnetic (HPEM) sources during the late 1990s raised the concern in the electromagnetic compatibility (EMC) community that they could be deployed for criminal purposes to interfere with the operation of modern electronic systems. It is well established that sufficiently intense electromagnetic fields can cause upset or damage in electronic systems and therefore, can affect almost every critical infrastructure (CI) that is based on information and communication technologies (ICT). This field of study was initially known as electromagnetic terrorism, but was changed to the more encompassing term of intentional electromagnetic interference (IEMI). This thesis is a contribution to the assessment techniques of the vulnerability of CIs against IEMI. In order to quantify their impact, the electromagnetic environment created by IEMI sources needs to be characterized, the susceptible components and subsystems of the CIs should be identified, and the expected disturbances have to be evaluated. We present a qualitative methodology to carry out the so-called IEMI audit of a facility. Given the complexity of the problem, it was decided that the vulnerability of an infrastructure should be evaluated in a qualitative manner by regarding the consequences of interrupting the normal provision of a service, the probability of occurrence of an IEMI attack, and the preparedness of the infrastructure to withstand an attack. An updated survey and classification of potential IEMI sources that were collected from a large number of scientific publications is presented. The sources have been classified according to their electromagnetic environment, their transportability, technological development, and cost level. The expected disturbances due to a high frequency illumination of representative cabling systems inside an office were studied through measurements performed using a plastic raceway containing several types of cables found in commercial buildings. The tests revealed that at low and intermediate frequencies, low voltage power cables are more susceptible compared to telephone or network cables. At high frequencies, the coupling is dominated by connector apertures and discontinuities and load unbalance. The applicability of the TL theory in evaluating differential mode signals in two-wire lines floating above a ground plane was studied through comparisons with full-wave simulations. The results showed that the validity of the TL theory is conditioned upon an electrically short distance between the differential wires, regardless of the distances above the ground plane. TL theory is also used to assess the effect of conductive and dielectric losses in the dispersion of injected IEMI signals along power and communication cables as a function of the propagation length. A TL model of the low voltage power cabling of the plastic raceway was developed and in order to validate the models, the numerical results were compared against measurements obtained using frequency and time domain techniques. General considerations and guidelines for the application of the TL theory for evaluating the overall transfer impedance of complex cable assemblies are given. The obtained simulation results were found to be in good agreement with the experimental data up to frequencies of about 500 MHz. Finally, an improved model for estimating the transfer impedance of a two-layer braided shield is also proposed and validated using experimental data.