The electromagnetic effects of a High Altitude EMP have been a threat for communications and installations, largely studied and documented between 1960 and 1990. During the last decade of the 20(th) century, a new kind of electromagnetic field representing a new threat, has been developed, the High Power Electromagnetic (HPEM) field. The paper will introduce and compare the main features of these two kind of electromagnetic fields, like the spatial coverage, the frequency spectrum, the time-domain behavior and the field intensities. It will also discuss effects related to these two phenomena on electrical and electronic installations and will compare the mitigation methods which have been developed and applied.