Using a clear, non-technical style, Professor Rohrlich discusses the two major theories of twentieth-century physics: relativity and quantum mechanics. Discussed conceptually and philosophically, rather than using mathematics, the philosophical issues raised show how new discoveries forced physicists to accept often strange and unconventional notions. He aims to remove the mystery and misrepresentation that often surround the ideas of modern physics and to show how modern scientists construct theories, so that the reader can appreciate their successes and failures and understand problems that are as yet unresolved.