The 20th century has been one of unparalleled scientific advances fuelled primarily by discoveries made by physicists. The 20th century also represents the life span of the American Physical Society (APS), and to celebrate its centenary the APS has prepared this book highlighting the seminal discoveries, with invited articles by some of the world's most eminent living physicists, including 12 Nobel Prize winners. The articles are mainly perspectives on discoveries written by those scientists closely associated with the original work, as well as covering future directions of research.