Understanding quantum mechanics matters because it is the engine that powers the universe. This engine is fuelled by a few simple principles, but the consequences of those principles are both profound and strange. In this richly illustrated book, quantum mechanics is explained using a finely balanced combination of words, diagrams and mathematics. The result is a tour of the most intriguing aspects of quantum mechanics, including Einstein's spooky action at a distance', Bell's inequality, Schroedinger's cat, Heisenberg's uncertainty principle, and de Broglie's matter waves. Supported by a comprehensive Glossary, Further Readings, and tutorial appendices, this is an ideal introduction to the mathematics of quantum mechanics.