The book examines the notion of impossible worlds. Although the notion of an impossible world is not novel, several misunderstandings and misrepresentations have resulted in philosophical confusion. To avoid such confusion, the analysis here differentiates between the logical, semantic, epistemological, and metaphysical roles of impossible worlds. The book's central hypothesis is that an understandable taxonomy of extreme positions about impossible worlds is required; it is not easy to hold the middle ground without taking the extreme positions seriously. These positions are compared in order to draw important lessons for logic, epistemology, and metaphysics. The Extent of Impossibility is essential reading for scholars in philosophy and especially for scholars and researchers focusing on metaphysics, logic and epistemology. Martin Vacek is a researcher at the Institute of Philosophy at the Slovak Academy of Sciences, where he serves as Vice-Director. He is also the Editor-in-Chief of Organon F: International Journal for Analytic Philosophy.