Index-based weather insurance is a major institutional innovation that could revolutionize access to formal insurance for millions of smallholder farmers and related individuals. It has been introduced in pilot or experimental form in many countries at the individual or institutional level. Significant efforts have been made in research to assess its impacts on shock coping and risk management, and to contribute to improvements in design and implementation. While impacts have typically been positive where uptake has occurred, uptake has generally been low and in most cases under conditions that were not sustainable. This paper addresses the reasons for this current discrepancy between promise and reality.