In physics there is a quest to find the mythical Grand Unified Theory. It was begun by Einstein and it is a belief that there exists a singular ultimate theory of Nature and physics. It has not been found. Its search consumes today's physics. It has seen radical ideas, novelties, research paths, experiments, and more to find it. It consumes legions of physicists worldwide. Here in this book, we will review discussion on this amazing mystery, epic treasure hunt, and search for ultimate understanding of Nature.