This book analyses the common law"s approach to retroactivity. The central claim is that when a court considers whether to develop or change a common law rule the retroactive effect of doing so should explicitly be considered and, informed by the common law"s approach to statutory construction, presumptively be resisted. As a platform for this claim a definition of "retroactivity" is established and a review of the history of retroactivity in the common law is provided. It is then argued that certainty, particularly in the form of an ability to rely on the law, and a conception of negative liberty, constitute rationales for a general presumption against retroactivity at a level of abstraction applicable both to the construction of statutes and to developing or changing common law rules. The presumption against retroactivity in the construction of statutes is analysed, and one conclusion reached is that the presumption is a principle of the common law independent of legislative intent....