The goal of this book is to collect, classify, and study human and machine solutions to some classic problems that have frequently been posed in mathematics, computer science, and artificial intelligence (AI). Overall, we classify and view these problems as AI problems because of their particular characteristics: they are nontrivial, at least for humans; they involve logic; they may involve search and memory; solutions to these problems involve certain techniques, such as deduction, inference, solving subproblems, and pattern recognition, which are applicable and extendable to other problems; they are by and large well-known and have survived the test of time; some of these problems have been used as a testbed for AI problem-solving techniques—for example, The 15 Puzzle, Mastermind, The Knight’s Tour Problem, The Red Donkey Puzzle, and Cryptarithms; and they are amenable for studying human problem-solving activity. The long-term, higher goal of our work is the development of a theory for problem solving.