Social Impact Bonds are a form of financing that aligns investor returns with social outcomes: investors only receive a return if the social outcome is achieved. Since Social Finance launched the first Social Impact Bond in September 2010 to reduce re-offending among short sentenced prisoners leaving Peterborough Prison, the concept has attracted considerable interest. There is, however, a long way to go before they are commonly used. Like any new approach, it will take a while for people to understand when and how to establish Social Impact Bonds. The purpose of this paper is to help commissioners consider how best to develop and procure Social Impact Bonds.