Albert Einstein and J. Robert Oppenheimer, two iconic scientists of the twentieth century, belonged to different generations, with the boundary marked by the advent of quantum mechanics. By exploring how these men differed - in their worldview, in their work, and in their day - this book provides powerful insights into the lives of two critical figures and into the scientific culture of their times. In Einstein's and Oppenheimer's philosophical and ethical positions, their views of nuclear weapons, their ethnic and cultural commitments, their opinions on the unification of physics, even the role of Buddhist detachment in their thinking, the book traces the broader issues that have shaped science and the world. Einstein is invariably seen as a lone and singular genius, while Oppenheimer is generally viewed in a particular scientific, political, and historical context. Silvan Schweber considers the circumstances behind this perception, in Einstein's coherent and consistent self-image, and its relation to his singular vision of the world, and in Oppenheimer's contrasting lack of certainty and related non-belief in a unitary, ultimate theory. Of greater importance, perhaps, is the role that timing and chance seem to have played in the two scientists' contrasting characters and accomplishments - with Einstein's having the advantage of maturing at a propitious time for theoretical physics, when the Newtonian framework was showing weaknesses. Bringing to light little-examined aspects of these lives, Schweber expands our understanding of two great figures of twentieth-century physics - but also our sense of what such greatness means, in personal, scientific, and cultural terms.