Einstein endorsed the view of Kaluza that gravity could be combined with electromagnetism if the dimensionality of the world is extended from 4 to 5. Klein applied this idea to quantum theory, laying a basis for the various modern versions of string theory. Recently, work by a group of researchers has resulted in a coherent formulation of 5D relativity, in which matter in 4D is induced by geometry in 5D. This theory is based on an unrestricted group of 5D coordinate transformations that leads to new solutions and agreement with the classical tests of relativity. This book collects together the main technical results on 5D relativity, and shows how far we can realize Einstein's vision of physics as geometry.