In this comprehensive study of Wittgenstein's modal theorizing, Raymond Bradley offers a radical reinterpretation of Wittgenstein's early thought. He argues that the Tractatus presents a view of the world in which possibilities are given an important ontological status. Contrary to most interpreters, Bradley contends that Wittgenstein's ontology is central to his enterprise, and not simply a by-product of certain of his views on language. On Bradley's reading, the Tractatus offers a version of modal realism. He further demonstrates the unexpected existence of deep differences both in content and aims between the logical atomism of Wittgenstein and that of Russell. A unique feature of Bradley's argument here is his reliance on Wittgenstein's notebooks, which he believes offer indispensable guidance to the interpretation of difficult passages in the Tractatus. Bradley then goes on to argue that Wittgenstein's account of modality - and the related notion of possible worlds - is in fact superior to any of the currently popular theories in this area.