This book is a treatment on the foundational kwledge of Nuclear Science and Engineering. It is an outgrowth of a first-year graduate-level course which the author has taught over the years in the Department of Nuclear Science and Engineering at MIT. The emphasis of the book is on concepts in nuclear science and engineering in contrast to the traditional nuclear physics in a nuclear engineering curriculum. The essential difference lies in the importance we give to the understanding of nuclear radiation and their interactions with matter. We see our students as nuclear engineers who work with all kinds of nuclear devices, from fission and fusion reactors to accelerators and detection systems. In all these complex systems nuclear radiation play a central role. In generating nuclear radiation and using them for beneficial purposes, scientists and engineers must understand the properties of the radiation and how they interact with their surroundings. It is through the control of radiation interactions that we can develop new devices or optimize existing ones to make them more safe, powerful, durable, or ecomical. This is why radiation interaction is the essence of this book.