This book provides engineers with a comprehensive understanding of nuclear reactor physics and neutronics, emphasizing the importance of simple models to validate complex computational results. It explains the rationale behind neutron slowing down and offers a straightforward method to evaluate the resonance escape probability in Fermi's 4-factors formula. The book includes exercises to assess the remaining three factors and demonstrates how to derive the diffusion approximation from the Boltzmann equation. It covers both one-group and two-group diffusion models, applying them to predict the reactivity of a nuclear reactor core. Special attention is given to the selection of the migration area. Additionally, the book addresses delayed neutrons, reactor kinetics, fission product poisoning, fuel evolution, fuel management, critical size, temperature effects, and xenon oscillations. Originally written for students, it contains 28 exercises with solutions provided in the appendix, making it an invaluable resource for both learning and practical application in the field.