First published in 1951, Enrico Fermi’s Elementary Particles remains a valuable guide for physicists and scholars. Fermi’s descriptions of the then-known particle universe and its nascent conceptual framework allow readers to glimpse the foundations of the field from the perspective of one of its most distinguished contributors. Over sixty years of research have provided answers to some of the questions Fermi poses in this book, but the biggest mysteries, regarding the origin and unification of forces, remain. As the high-energy physics community analyzes the results from ongoing experiments, such as those at the Large Hadron Collider, this historic work will be of interest to researchers, academics, and students. A new foreword by Yale University physicist Thomas Appelquist provides an engaging update and gives the work historical context.