This Article examines the function of reasonable investment-backed expectations as a factor in modem takings jurisprudence. Part II of the Article explores the origins of the doctrine. Part III examines how the phrase has been defined and interpreted since the Penn Central decision. This part separately discusses the two components of the doctrine ("reasonable" and "investment-backed"). Part IV examines recent holdings applying this doctrine, emphasizing the most recent takings decision to come from the Supreme Court, Lucas v. South Carolina Coastal Council. The Article concludes with a discussion of the future of this factor in takings jurisprudence.