Extra dimensions — beyond space and time — are the best methods for unifying gravity with particle physics. The basic extension is to five dimensions (5D), as in the induced-matter and membrane theory. This descriptive text gives an up-to-date account of the classical and quantum consequences of 5D physics. It includes topics that range from Einstein's original theory of relativity to modern views on matter. The book is mathematically precise and focuses on new ideas which appeal to readers. Examples of new ideas are: The big-bang universe, which is curved by matter in 4D, may be viewed as a smooth and empty world in 5D; the uncertainty of quantum interactions in spacetime may be regarded as the consequence of deterministic laws in higher dimensions. This book will interest people who think about the 'meaning of things'.