This is a study of atomic nuclei, the properties of which find applications in diverse fields such as medicine and the generation of nuclear power. The explanations of nuclear structure and nuclear reactions in terms of constituent particles led to the discovery of new unstable particles culminating in the unified theory of electromagnetic and weak-nuclear interactions. In this new edition the author has updated the information and included a section on the relevance of recent discoveries to cosmology. Advances in accelerator and detector technology are noted and there is an account of nuclear reactions. Mathematical material is mostly to be found in the appendices. This book may be used as a first student text for courses in nuclear physics.