This is a book about the history of the science of inertia. Nobody denies the existence of the forces of inertia, but they are branded as "fictitious" because they do not fit smoothly into modern physics. Named by Kepler and given mathematical form by Newton, the force of inertia remains aloof because it has no obvious local cause. At the end of the 19th century, Ernst Mach bravely claimed that the inertia of an object was the result of its instantaneous interaction with all matter in the universe. Many other well-known physicists, including Aristotle, Galileo, Descartes and Einstein, are shown to have tackled this difficult subject. The book also concentrates on inertia research in the 20th century, taking place under the shadow of general relativity, which is seen as uncomfortable with Mach's principle. A Newtonian paradigm, based on action-at-a-distance forces, is discussed throughout the book, allowing the revival of Mach's principle as the only coherent explanation of the inertia forces which play such an important role in the laboratory and in the cosmos.