In this appendix, we collect together some basic material that readers should know before studying this book. The material includes sets, enumeration, and probability. In practice, we have found that students have often already been exposed to this material and only need a quick review, so we did not want to include another introductory chapter delaying the study of our main topics. We cover sets, subsets, universal sets, unions,intersections and complements, Cartesian products, power sets, and rules such as De Morgan's laws. For enumeration, we cover the addition and multiplications rules and special cases such as permutations and combinations. For probability, although our coverage is less detailed than other chapters, we actually cover more than we need for the rest of the book, including conditional probability and Bayes's theorem. Most readers will be comfortable just using this chapter as a reference or review.