The content of this chapter is mainly methodological. The material should be regarded as a peculiar theoretical introduction to the rest of the book. Here, we will formulate the key methodological concepts and ideas that form the basis of the general FEL theory. Something similar has already been done in Part 1. However, the material of Part 1 covers only the most abstract aspects of the theory. In contrast, this chapter will cover the problems that are characteristic for hierarchic electrodynamic oscillation-wave-resonant systems. They will be used for physical analysis of different types of FEL models.