The scenario of the brain in a vat, first aired thirty-five years ago in Hilary Putnam's classic paper, has been deeply influential in philosophy of mind and language, epistemology, and metaphysics. This collection of new essays examines the scenario and its philosophical ramifications and applications, as well as the challenges which it has faced. The essays review historical applications of the brain-in-a-vat scenario and consider its impact on contemporary debates. They explore a diverse range of philosophical issues, from intentionality, external-world scepticism, and the nature of truth, to the extended mind hypothesis, reference magnetism, and new versions of realism. The volume will be a rich and valuable resource for advanced students in metaphysics, epistemology, and philosophy of mind and language, as well as anyone interested in the relations between language, thought and the world.