From the results discussed through the preceding chapter one realizes that at a fundamental level, basic physical processes, like interference effects, lasing action, quasi-phase matching conditions, or the coupling between electromagnetic waves and the collective motion of electrons, to name just a few, are essentially the same in both periodic and aperiodic optical systems. Thus, in the theory of aperiodic systems the role of order itself is what it is all about (at least at the current state of knowledge in the field). Accordingly, we shall open this chapter by considering three main queries: