This book represents a collection of 30 selected papers from the work of John W. Cahn. Dr. Cahn is Senior Fellow at the Materials Science and Engineering Laboratory of the National Institute of Standards and Technology, and is widely recognized as a founder of modern theory and thought in materials science. The range of his research included kinetics and mechanisms of metallurgical phase changes, surfaces, interfaces, defects, quasicrystals, thermodynamics, and other areas impacting the fundamental understanding of materials science. Each paper includes a 2-4 page review of the impact and historical perspective of the work. This is an important collection for students, instructors, and scientists interested in materials science.