ANS has reproduced the long out-of-print Nuclear Reactor Theory, thanks to the Department of Energy, which holds the book’s copyright. The book explains the most important physical concepts and mathematical methods commonly used in predicting the behavior of neutrons in nuclear reactors. It serves as an excellent introduction to reactor theory for physicists, mathematicians, and engineers and was written by two pioneers in the nuclear field. This seminal work is a must for any student of nuclear theory.