While most of the emphasis has been focused on the radiated threat represented by intentional electromagnetic interference, it is clear that the threat from conducted disturbances should not be neglected. Conducted threats include those that are produced by the cable coupling of radiated fields and from the direct injection of conducted disturbances into the wiring of a building. It is well known that in the majority of cases, cables provide the most efficient means of transporting potentially damaging energy into equipment. The first part of this paper describes how electrical disturbances can impact electronic equipment inside a building through the power and earthing circuits. Measured data are summarized to illustrate how electrical signals propagate through the power and earthing circuits from the outside to the inside and what levels of signals create problems with electronic equipment. With this understanding of the impact of transient waveforms on electronic equipment and how these disturbances can propagate throughout a building, calculations are then used to evaluate different types of conducted threats to buildings