Erwin Freundlich (1885-1964) was a renowned astronomer and close working associate of Albert Einstein. In this volume Freundlich provides a concise explanation of the ideas and theories which led to the general theory of relativity. Remarkably lucid and readable considering the difficulty of its subject matter, the text is also notable for containing a preface written by Einstein himself. A fascinating book, it will be of value to anyone with an interest in general relativity and the history of science. This English translation by Henry L. Brose was first published in 1920, and the German original in 1916.