The twentieth century has brought enormous changes in the physicist's understanding of the fundamental nature of the physical world. These changes were ushered in the century's first decade, with the advent of relativity and quantum theory. With advancing knowledge, the mystery surrounding the ultimate nature of the physical world has deepened, not lessened, and the search for the Holy Grail of a 'Grand Unified Theory' or 'Theory of Everything' continues. This book reveals, in considerable detail, the concepts that have arisen as a result of that search -- the 'state of art'.