Category mistakes are sentences such as "Green ideas sleep furiously", "Saturday is in bed", and "The theory of relativity is eating breakfast". Such sentences strike most speakers as highly infelicitous but it is a challenge to explain precisely why they are so. Ofra Magidor addresses this challenge, while providing a comprehensive discussion of the various treatments of category mistakes in both philosophy of language and linguistics. The phenomenon of category mistakes is particularly interesting to both these fields because a plausible case can be (and has been) made for explaining it in terms of syntax, semantics, and pragmatics - making it a fruitful case for exploring the relations between and nature of these three fundamental realms of language. Category Mistakes follows this division. After an introduction which explains the aims and motivations for the project and provides a brief historical survey of the (modern) treatment of category mistakes in each of philosophy, linguistics, and computer science, Magidor discusses four approaches in turn: first, the syntactic approach, which maintains that category mistakes are syntactically ill-formed; then two semantic approaches, though ones that appeal to different semantic facets: the meaninglessness view, which maintains that category mistakes are meaningless, and the MBT view, according to which category mistakes are meaningful but truth-valueless; and finally the pragmatic approach, according to which category mistakes are syntactically well-formed, meaningful, truth-valued but nevertheless pragmatically inappropriate. Magidor argues that the first three approaches ought to be rejected, and in the final chapter addresses the main challenge by developing and defending a particular version of the pragmatic approach: a presuppositional account of category mistakes.