Relativity is the most important scientific idea of the twentieth century. Albert Einstein is the unquestioned founder of modern physics. His Special and General theories of Relativity introduced the idea to the world. In this classic short book he explains clearly, using the minimum amount of mathematical terms, the basic ideas and principles of his theory of Relativity. Unsurpassed by any subsequent books on Relativity, this remains the most popular and useful exposition of Einstein's immense contribution to human knowledge.