The ultimate questions of how, when, and why the Universe came into being have fascinated thinkers for centuries. What have traditionally been philosophical and theological problems are now bing addressed by the mathematical speculations of a new generation of scientists, who have begun to take seriously the idea that there is an ultimate theory of the physical world which it is within our power to discover. If such a theory were discovered it would reveal the complete form of the most basic Laws of Nature, from which all others follow as particular simple cases. This book lays out and examines the different ingredients that are required to claim a full understanding of the physical universe that we observe, and shows how their status is being dramatically changed in modern physics and what aspects of them can or cannot be explained by a "Theory of Everything". Modern physicists believe they have stumbled upon a key which unlocks the mathematical secret at the heart of the Universe: a discovery that points towards a "Theory of Everything" - a single all-embracing pricture of the Laws of Nature from which the inevitability of all things seen must follow with unimpeachable logic. Is this confidence well-founded? Will our scientific theories one day tell us everything that was, is, and is to come about the Universe? These modern "Theories of Everything" claim they will tell us everything about the Universe in which we live. But will they?