This book covers introductory subjects including fundamental principles of nuclear reactions with neutrons, fundamentals of nuclear fission chain reactions, basic concepts of criticality, and static characteristics based on diffusion approximation in neutron transport. The chapters address topics ranging from neutron moderation from fission to thermal energy ranges and heterogeneity effects in neutronics. Readers will find elementary and qualitative descriptions and also mathematical expressions including approximations, derivations and analytical solutions for an understanding of the basic principles of nuclear reactor physics. This book is part of a series entitled An Advanced Course in Nuclear Engineering and provides an accessible introduction to the core discipline of nuclear engineering: nuclear reactor physics. It will therefore appeal to engineers in nuclear engineering as well as to university students and others seeking to learn entry-level reactor physics.