This monograph is concerned with two foundational principles of English property law: the principle of relativity of title and the principle that possession is a source of title. It is impossible to understand the relationship between possession and ownership in English law unless one has a sound understanding of these principles. Yet the principles have been interpreted in different ways by judges, practitioners, and academics. The volume seeks to illuminate this area of law by addressing four questions. What is possession? What is the nature of the title acquired through possession? What are the grounds of relativity of title? And, what is the relationship between relativity of title and ownership? Drawing on the analysis of the law concerning relativity of title and the acquisition of proprietary interests through possession, the author also implies that the architecture of land law and the law of personal property have many similarities.