This book studies the categories of subject matter protected by the main intellectual property (IP) regimes, including: the work (protected by copyright law), the invention (protected by patent law), the sign (protected by trade mark law) and the design (protected by designs law). The book makes sense of the law as it exists and argues for its clarification along certain lines, taking international law into account. The book focuses on the law's conception of the categories in the abstract and the courts' conception of individual subject matter in practice. In undertaking a close analysis of each category, it shows that when properly conceived the categories are different in their constitutive aspects and that their protection by different property regimes is therefore appropriate. In discussing the basic subject matter of each form of IP protection, the book moves beyond the question 'what is an X?' toconsider also how the courts conceive individual subject matter in their capacity as 'X'. It also considers non-legal theories of the subject matter categories, the challenges faced by IP law in the current technological age, and the appropriate response to these challenges. This is an invaluable guide to a central aspect of IP law, offering clear theoretical insights and practical guidance for academics and practitioners alike.