${\displaystyle N}$ body problem is the fundamental problem of the modern description of Nature. In this chapter we have shown through various examples the variety of matter arrangement one can encounter in Nature. We have noticed that, in general, among the four fundamental interactions (strong interaction, weak interaction, electromagnetic interaction, gravitation), gravitational and/or electromagnetic interactions are relevant for the description of system of super nuclear scales that are our interest in the rest of the book. In the next two chapters, we will present the description of gravitational and electromagnetic interactions.