OECD/UNDP – Tax Inspectors Without Borders. Annual Report 2017/18. This Annual Report from Tax Inspectors Without Borders (TIWB) covers the period May 2017 to April 2018. TIWB’s practical and results-oriented approach to supporting domestic resource mobilisation is proving increasingly relevant in a fast moving international environment. TIWB is contributing to the United Nations’ Financing for Development agenda, and supporting progress towards attaining the Sustainable Development Goals (SDGs). It is also underpinning the Base Erosion and Profit Shifting (BEPS) actions, strengthening developing countries ability to effectively tax multinational enterprises (MNEs), while offering increased certainty and predictability to taxpayers. TIWB increasingly operates in close partnership with a diverse range of stakeholders and partners. Demand for TIWB is growing. There are 29 programmes currently operational and 7 have been completed, together exceeding the target of 35 programmes by April 2018 set by the TIWB Governing Board. Over 20 programmes are in the pipeline. New South-South opportunities are being identified, with India, Nigeria, and South Africa now offering expertise. These developments are, in part, due to increased active participation from Partner Administrations (those providing experts), with 11 countries deploying their serving tax officials and a United Nations Development Programme (UNDP) managed roster of 40 tax audit experts up and running. To date, USD 414 million in increased tax revenues is attributable to TIWB and TIWB-style support offered in partnership with the African Tax Administration Forum (ATAF) and the World Bank Group (WBG). TIWB represents excellent value for money with over USD 100 in additional tax revenues recovered for every USD 1 spent on operating costs. Whilst revenue impact is important, in the last year TIWB has gathered evidence of other long-term outcomes, including on skills transfer, organisational change and taxpayer compliance. The TIWB Secretariat has developed new tools to help with the measurement challenge. In 2017, an Experts’ Roundtable and a Stakeholders’ Workshop, involving stakeholders from 28 countries and 6 international and regional organisations, gathered lessons on how TIWB’s unique role could be strengthened and how the target of 100 tax expert deployments by 2020 should best be achieved. A mentorship programme was proposed. Other lessons include the finding that TIWB programmes with full access to taxpayer information have advantages over anonymised case reviews and can help with tax reforms by illuminating possible legislative shortcomings in international taxation. The importance of a whole-of-government approach by Partner Administrations, which could improve the efficiency of expert deployment processes with institutionalised funding arrangements, was also highlighted. The partnership between the Organisation for Economic Co-operation and Development (OECD) and UNDP, which delivers TIWB, is becoming stronger with an agreed division of labour. UNDP country offices are able to facilitate national-level discussions on domestic resource mobilisation (DRM), raise awareness and build national support for TIWB programmes. The TIWB Secretariat has launched its first e-newsletter and community of practice for its Experts. TIWB has also updated its multilingual website. The year ahead will see the TIWB Secretariat pursue the implementation of the 2016- 2019 Objectives (Annex A). Priorities will include cementing partnerships with regional tax organisations, expanding the scope of TIWB to new areas such as tax and crime, continuing to build South-South programmes and building a pool of industry expertise to assist developing countries address audit challenges in key business sectors. A major international conference on TIWB and possible future directions is being considered for 2019.