The working title of this book was “Combinatorics 18.315.” In the private language of the Massachusetts Institute of Technology, Course 18 is Mathematics, and 18.315 is the beginning graduate course in combinatorial theory. From the 1960s to the 1990s, 18.315 was taught primarily by the three permanent faculty in combinatorics, Gian-Carlo Rota, Daniel Kleitman, and Richard Stanley. Kleitman is a problem solver, with a prior career as a theoretical physicist. His way of teaching 18.315 was intuitive and humorous. With Kleitman, mathematics is fun. The experience of a Kleitman lecture can be gleaned from the transcripts of two talks. Stanley's way is the opposite of Kleitman. His lectures are careful, methodical, and packed with information. He does not waste words. The experience of a Stanley lecture is captured in the two books Enumerative Combinatorics I and II, now universally known as EC1 and EC2. Stanley's work is a major factor in making algebraic combinatorics a respectable flourishing mainstream area.