In an overridingly urban world, cities are key sites and crucial actors for sustainable development. Yet, in developing countries, they still often lack the resources that would enable them to tackle the challenges posed by sustainability. This has prompted an increasing level of advocacy in favour of bolstering their financing capacities (New Urban Agenda 2016, §139). In tandem, the new paradigm of international development financing is emerging. Geared to attracting the private sector to actors that are reputedly financially fragile or little-known, this shift is driving an interest in instruments, such as guarantees, that can both mitigate risk and leverage private funds (Addis Ababa Action Agenda, 2015, §34). At the crossroads of these two logics, guarantees that back developing city financing seem to offer a promising way forward. They have been in use since the 1980s to support the financing of states and companies, but it was only a few years ago that donors envisaged using them to finance municipalities. These initiatives, however, are still few and far between and the lessons they offer have not been sufficiently exploited to enable their real potential to be assessed.