Jos Vaessen is Chief Evaluation Officer and Senior Adviser at the Independent Evaluation Group at the World Bank. Since 1998 he has been involved in evaluation research activities, first as an academic and consultant to bilateral and multilateral development organizations and from 2011 to 2016 as a lead evaluator at UNESCO. Between 2016 and 2024 he has worked at the World Bank's Independent Evaluation Group as evaluation adviser. He established and led IEG's Methods Team (2016-2022) and in collaboration with other colleagues and institutional partners, contributed to establishing the Global Evaluation Initiative, a global partnership to strengthen M&E systems and capacities. In 2024-2025 Jos was Principal Advisor at the Office of Evaluation and Oversight at the Inter-American Development Bank. Jos has taught evaluation-related courses at the University of Antwerp (2002-2010), Maastricht University (2010-2012), the International Program for Development Evaluation (2016-present) as well as various other institutions.

Jos has been author of several internationally peer-reviewed publications, including three books. Notable examples of his publications are: Impact evaluations and development – NONIE guidance on impact evaluation (2009, with Frans Leeuw; World Bank), Mind the gap: perspectives on policy evaluation and the social sciences (2009, with Frans Leeuw; Transaction Publishers), Dealing with complexity in development evaluation: a practical approach (2015, with Michael Bamberger and Estelle Raimondo; SAGE Publications), Evaluation of international development interventions: an overview of approaches and methods (2020; with Sebastian Lemire and Barbara Befani; World Bank). Jos regularly serves on reference groups of evaluations for different institutions.