The aim of the book is to provide a clear, concise and self-contained discussion of both the structure of the theory of special relativity and its physical content. The point of view is that of a practising physicist who uses relativity daily: relativity is a branch of physics and is regarded as being neither mathematics nor philosophy. Particular care has been taken to elucidate those difficulties, conceptual rather than mathematical, which invariably snare the unwary or inexperienced. The material is liberally illustrated with real examples and problems drawn from both high energy physics and from astrophysics.