We find ourselves in an era of unprecedented growth in the development and use of so-called “orphan” drugs to treat rare diseases, which are poised to represent more than one-fifth of pharmaceutical expenditures by 2022. This widespread use has been facilitated by legislative and regulatory incentives in both the United States and abroad, yet US payers and health systems have not yet made a concerted effort to understand whether and how rare diseases require special considerations on their part and how to adapt traditional methods of health technology assessment and economic evaluation to accommodate these situations. In this article, we explore the general ethical dilemmas that rare diseases present, steps taken by health technology assessment bodies worldwide to define the level of rarity that would necessitate special measures and the modifications to their assessment and valuation processes needed, and the contextual components for rare-disease evaluation that lie outside of the assessment framework as a guide to US decision makers on constructing a formal and relevant process stateside.