This chapter takes as its point of departure a significant book that has inspired media and communication studies even though it treats issues of information and communication from a mathematical and technical angle. This chapter introduces Claude Shannon and Warren Weaver’s communication theory and its central concepts, which have been especially important in the field of media and communication studies, including the now-common terms information, selection, uncertainty and the (im)probability of communication. This chapter first reviews the mathematical theory of communication, as formulated by Shannon, and the additions provided by Weaver; it then briefly presents the criticism against this theory and how the theory has been used and further developed in fields such as cybernetics and systems theory.