This definitive comparative treatise provides a clear framework for understanding both the theory and practice of patent law. Grounded in the relevant international treaties such as the Agreement on Trade-Related Aspects of Intellectual Property Rights (TRIPS), the book identifies international obligations and draws out the common principles shared by patent systems across Europe and North America. Beginning with a historical perspective, the book provides an analytical overview of patent law as a dynamic system shaped by digital transformation, and traces how the foundational doctrines of patent law have evolved and adapted to the realities of Industry 4.0, digital transformation, and AI innovation. In addition to delivering doctrinal analysis of the fundamentals of patent law, such as eligibility, novelty, inventive step, infringement, claim scope, remedies and defenses, the book also presents research questions inviting readers to examine how platform ecosystems, servitization, and AI tools challenge how these fundamentals are approached. It situates novel developments within the structure of traditional patent principles and concludes with forward-thinking research questions designed to stimulate future inquiries. The Law and Theory of Patents is a crucial resource for scholars and students of patent law, innovation policy, and intellectual property (IP) law. Patent practitioners, judges, and regulators will also benefit from its comparative guidance and actionable insights into the complex future of patent law.