As a response to the unsustainability of current global financial systems, parallel sustainable monetary systems are being developed by civil society groups and non-governmental organisations (NGOs), informed by ecological economics perspectives on development, value, economic scale and growth. These parallel systems of exchange (or grassroots complementary currencies) are designed to promote sustainable development, by localising economic development, building social capital and substituting for material consumption, valuing work which is marginalised in conventional labour markets, and challenging the growth-based monetary system. However, this international movement towards community-based ecological economic practices, is under-researched. This paper presents new empirical evidence from the first international study of the scope and character of grassroots currencies. It identifies the diversity, scale, geography and development trajectory of these initiatives, discusses the implications of these findings for efforts to achieve sustainable development, and identifies future research needs, to help harness the sustainability potential of these initiatives.