Tax capacity building to support developing country participation in international rule-making and implementation of international standards is a key priority for the OECD and a significant part of our work programme, highlighting the OECD commitment to international tax cooperation. Through their contributions to domestic resource mobilisation and the realisation of the Sustainable Development Goals (SDGs), OECD capacity building activities have a concrete impact in a global economic context in which developing countries face multiple challenges. Importantly, these activities comprehensively reinforce the capacities of tax policymakers and tax administrations, recognising that the additional revenues needed to finance the SDGs will require a range of different measures and tax policy reforms beyond the reform of international corporate income taxation. This report describes the work of the OECD to assist developing countries to improve their tax systems, covering the full range of assistance on tax policy and administration issues to counter corporate tax avoidance, combat illicit financial flows and mobilise domestic resources to achieve the SDGs.