Private law is a foundational part of the law. But what is it, and why does it take the form it does? Many authors offer theoretical accounts, drawing on very different intellectual traditions: some base private law on economics, some on social policy, some on particular schools of moral philosophy, some on the traditions of the common law. We do not lack theory: if anything we have too much. Worse, many writers have sought to theorise individual parts of private law in isolation from the remainder, so that the law's inter-connectedness and seamless fluidity have begun to fade from view. The forest of theoretical literature has become vast and tangled. This book provides a path through the forest. It argues that the theory of private law is a coherent topic of enquiry, and that the many theoretical contributions to it can be seen as rival attempts to answer its core questions, which are: 'How do we characterise the main institutions of private law?' (the 'What' question); and 'How do we justify those institutions?' (the 'Why' question). The aim of the book is to survey this broad field, to explore common themes, and to guide its readers through the various issues and debates to date.