Book excerpt: This book attempts to explain why 'string theory' may provide the comprehensive underlying theory that describes and explains our world. It is an enthusiastic view of how compactified string/M-theories (plus data that may be reachable) seem to have the possibilities of leading to a comprehensive underlying theory of particle physics and cosmology, perhaps soon. We are living in a hugely exciting era for science, one during which it may be possible to achieve a real and true understanding of our physical world.