Information theory plays a fundamental role in quantifying the side-channel leakage by information-theoretic measures to be exploited by various distinguishers. Applications include the construction of new side-channel distinguishers, the generic information-theoretic bounds, and the formal security proofs. The main information measures include mutual information and \(\alpha \) -information, where the latter generalizes the former by following the Rényi’s \(\alpha \) -entropy paradigm. This chapter presents the information-theoretic formalization and evaluation of side-channel attacks. First, two distinguishers are studied, namely, mutual information analysis (MIA) and inter-class information analysis (IIA), as well as the role of cumulants in high-order attacks. Then, \(\alpha \) -information theory is introduced with several formal definitions, along with the generalized Fano’s inequality. The information-theoretic bounds and connections with the attacks are detailed in the next chapter.