Countries’ capacity to raise sufficient revenue of their own is critical for sustainable development. Yet developing countries face many hurdles in increasing their tax‑to‑GDP ratios. This chapter illustrates how development co-operation offers large, but largely untapped, potential for supporting tax system reform. A range of well‑designed and co-ordinated development co-operation approaches, from budget support to technical assistance, have had positive results, including in the most challenging of contexts, such as Afghanistan. Technical assistance is becoming more innovative, as the Tax Inspectors Without Borders initiative, currently being piloted by the OECD shows. International co-operation is also key in ensuring that developing countries do not lose much-needed revenue to emerging global challenges, such as the taxation of multinational enterprises.This chapter also includes an opinion piece by Ngozi Okonjo-Iweala, Nigeria’s Co‑ordinating Minister for the Economy and Minister of Finance, on the importance of realising the potential of taxation.