This book comes out of a study prepared for the Space Technology and System Development Commission of the International Academy of Astronautics, and gives an expert, straightforward, and complete outlook on the uses of nuclear energy applied to space missions. Starting from fundamental physics, Chapter 1 explains the advantages of nuclear energy and explores the performance limits of nuclear propulsion in terms of specific impulse, thrust, power, and mass. Following chapters discuss the tremendous accomplishments of the past and moves into more current technology. High-power electric propulsion of all types is extensively covered. These chapters shows how nuclear power can be engineered into a propulsion system now, not in ten or twenty years.