This book focuses on the search for a universal theory of life. It is concerned with the history of attempts to develop such a theory, diagnosing why these efforts have thus far been unsuccessful, and determining what is required to forge ahead and successfully pursue such a theory. It is of course possible that the diverse phenomena of life lack an objective natural unity, and hence that no such theory will ever be forthcoming. Indeed, this view has become popular among some biologists and many philosophers of biology. One of the central themes of the book is that skepticism about the prospects of universal biology is not only very premature but also potentially self-fulfilling: One does not want to short-circuit the potentially successful pursuit of universal biology by rejecting it out of hand.