With more than 75 years of combined working experience in the area of reactor physics and safety, the intention of the authors of this monograph is to provide a practical book on reactor physics, particularly for the safe operation of aged CANDU reactors, with minimal mathematics or equations. The book gives a glimpse of first principles and their engineering application in reactor physics, for those who are interested in or are working in the Canadian nuclear industry. The book is also ideal as a reference for physicists, operators, regulatory staff, and for those who need to interact with reactor physicists at CANDU sites, nuclear laboratories, institutes, universities, or engineering companies.