This book provides a lively and visual introduction to Einstein's theory of relativity. It brings to life the excitement of this fascinating subject, for an audience including young people at school (post-16) and the general public with an interest in modern physics. It is different from existing books in that is uses many diagrams and simple equations (the reader is carefully guided through them), and richly rewards the reader with beautiful mathematical and physical insights. It begins by introducing spacetime, in the familiar context of low velocities. It then shows how Einstein's theory forces us to understand time in a new way. Paradoxes and puzzles are introduced and resolved, and the book culminates in a thorough unfolding of the relation between mass and energy. The book draws on the author's many years of experience in writing articles and reviews for a non-expert readership, and presenting physics to school pupils.