This technical report outlines the way in which electricity is now described as a product. Particularly, in Europe and several other areas, for example Brazil and Argentina, as well as in some states in the United States of America. It is, however, rather a unique product because of its intangible and transient nature. Strictly, it is a product that exists only for an instant at a given point of delivery, comes into existence at the same instant at which it is being used and is replaced immediately by a new product with rather different characteristics. Its characteristics are different at each separate point of delivery. Moreover, it is a product whose quality depends not only on the elements that go into its production, but also in the way in which it is being used at any instant by the equipment of multiple users. Therefore, the quality control that is possible for more tangible and concrete products is not applicable in the case of electricity. All that can be attempted is some control of the conditions under which it is produced, transmitted and distributed and those under which it is used. In particular, the capacity of utilisation equipment to impinge on the quality of electricity, including that delivered to other equipment, must be recognized. Electrical equipment has become increasingly complex in terms of the functions it fulfils and the way in which it interacts with other electrical equipment. Frequently, that interaction takes place through the medium of the electricity network, which is the common energy source for all the equipment. It arises because the network, intended to be a common energy source, also provides a conducting path interlinking all equipment. In effect, the electromagnetic phenomena arising from the behaviour of utilisation equipment are superimposed on the other characteristics of the electricity supply, and become part of the product that is delivered to the system user. They are joined also by phenomena arising from atmospheric and other external events and from the intrinsic response of a large electricity system to such events.