Nature-based solutions (NBS) face distinct policy and financing barriers that differentiate them from conventional grey infrastructure. The lack of evidence on their efficacy and co-benefits poses an almost intractable challenge to public authorities and private businesses intent on justifying NBS over their conventional counterparts. For many NBS, this evidence will not be forthcoming in time for their urgent scale up. Moreover, entrenched institutional, regulatory, and financial factors or ‘grey path dependency’ inadvertently enable conventional infrastructure. To add to the challenges, the public-good nature of most NBS means few bankable projects in contrast, for example, to private investments in renewable energy.