This book presents the long sought after quaternion form of the Dirac equation. Recent developments in understanding quaternion differentiation by the author of this book have produced a rewrite of the Dirac equation as a quaternion equation. The quaternion Dirac equation predicts parity violation by the weak force and explains how neutrinos can be both massless and oscillate between generations. We also have a significant step towards the unification of general relativity and quantum field theory. This book is a monumental triumph of work by many individuals over almost a century. It will be of interest to students and advanced theorists alike.