Timothy Williamson is one of the most influential living philosophers working in the areas of logic and metaphysics. His work in these areas has been particularly influential in shaping debates about metaphysical modality, which is the topic of his recent provocative and closely-argued book Modal Logic as Metaphysics (2013). The present book comprises ten essays by metaphysicians and logicians responding to Williamson's work on metaphysical modality. The authors include some of the most distinguished philosophers of modality in the world, as well as several rising stars. Each essay is followed by a reply by Williamson. In addition, the book contains a major new essay by Williamson, 'Modal science, ' concerning the role of modal claims in natural science. This book was originally published as a special issue of the Canadian Journal of Philosophy