The aim of this book is to show how a full understanding of units and dimensions provides a simple basis for explaining aspects of physical description that otherwise may seem confusing or mysterious. The way units and dimensions behave-in particular, the way their properties influence the mathematical description of physical behavior-makes sense only if certain basic notions and conventions are recognized. Once these are recognized, the strange quirks and uncanny powers of units and dimensions at once become easy to comprehend and simple to control.