On the 50th anniversary of Yang-Mills theory, this invaluable volume looks back at the developments and achievements in elementary particle physics that ensued from that beautiful idea. During the last five decades, Yang-Mills theory, which is undeniably the most important cornerstone of theoretical physics, has expanded widely. It has been investigated from many perspectives, and many new and unexpected features have been uncovered from this theory. In recent decades, apart from high energy physics, the theory has been actively applied in other branches of physics, such as statistical physics, condensed matter physics, nonlinear systems, etc. This makes the theory an indispensable topic for all who are involved in physics. An international team of experts, each of whom has left his mark on the developments of this remarkable theory, contribute essays or more detailed technical accounts to this volume. These articles highlight the new discoveries from the respective authors' perspectives. The distinguished contributors are: S Adler, P van Baal, F A Bais, C Becchi, M Creutz, A DeRujula, B S DeWitt, F Englert, L D Faddeev, P Hasenfratz, R Jackiw, P van Nieuwenhuizen, A Polyakov, R Stora, S Weinberg, F Wilczek, E Witten, C N Yang. Included in each article are introductory and explanatory remarks by the editor, G 't Hooft, who is himself a major player in the development of Yang-Mills theory.