The fundamental and very important property of inertia has never been well understood. This book shows how inertia has puzzled many scientists such as Galileo and Mach, and then presents a new theory that explains inertia for the first time, and also predicts galaxy rotation without dark matter, cosmic acceleration and some other anomalies. Further evidence for, and tests of, the theory are presented and exciting applications such as new inertial launch methods and the theoretical possibility of faster than light travel will be discussed. To allow readers to use the theory themselves, some simple maths is included, and to help explain the points made, there are numerous cartoons by the author.