This chapter contains two appendices related to the topics presented in this book. The first appendix contains some notions of axiomatic set theory. To some students it is not clear what is a set and what is a class. So it was often necessary to explain them what naive set theory and Russel’s paradoxes are, and how these lead to axiomatic set theory. We briefly describe ZFC, Grothendieck universes, and NBG, which is the axiomatic set theory most suitable in category theory. In the second appendix we quickly explain cardinal numbers and ordinal numbers. In particular we present the main operations on cardinal numbers (addition, multiplication and exponentiation) and their properties, the order on the class of cardinal numbers, and the Cantor-Schröder-Bernstein Theorem. As far as ordinals are concerned, we treat initial segments of well-ordered sets, successor ordinals, limit ordinals, the arithmetic of ordinal numbers (addition, multiplication and exponentiation), and Cantor normal form.