The reader should resist the temptation to skip the £rst three sections of this chapter. They may appear boring and complicated, but there is no way around them. These sections provide a rigorous set of basic de£nitions, concepts, fundamental laws, and postulates. They serve as the basis for the general hierarchic theory of relativistic plasma-like systems developed later in the book, including the high-current FELs. Therefore, summon up the patience and read ahead.