Jos Vaessen (Ph.D. Maastricht University) is Principal Evaluation Specialist at the Internal Oversight Service of UNESCO in Paris and lecturer at Maastricht University, The Netherlands. After completing his M.Sc. in 1997 (Wageningen University) and prior to starting his current position at UNESCO in 2011, he has been involved in research, teaching and evaluation activities in the field of international development at Antwerp University and, more recently, Maastricht University. Over the last fifteen years or so, he has worked for several multilateral and bilateral international organizations mostly on evaluation-related assignments. His fields of interest include: theory and practice evaluation, impact evaluation, rural development and environment. In addition to managing and conducting evaluations Jos regularly serves on reference groups of evaluations of different organizations. He has been (co-) author of more than 30 publications, including three books. Recent publications include: Impact evaluations and development – NONIE guidance on impact evaluation (2009, co-author, with F. Leeuw; NONIE), Mind the gap: perspectives on policy evaluation and the social sciences (2009, co-editor,with F. Leeuw; Transaction Publishers), Dealing with complexity in development evaluation: a practical approach (2015, co-editor with M. Bamberger and E. Raimondo; SAGE Publications).