In Memory of Siem Vaessen
It is with deep sadness that the International Aid Transparency Initiative (IATI) shares the news of the passing of Siem Vaessen.
Siem was a dedicated and deeply valued member of the IATI community. He first became involved in IATI in 2010 and, that same year, co-founded the technology company Zimmerman & Zimmerman. Over the years that followed, he became widely respected for his expertise and commitment to expanding the access, use and value of open data.
Siem Vaessen delivering remarks at the IATI Technical Advisory Group meeting in Dar es Salaam, 6-9 March 2017
Siem and his team first helped deliver the pilot for Open Aid NL, the Dutch government’s transparency portal providing IATI data on official development assistance. This work reflected an innovative approach to technology that would become a hallmark of his contribution to the IATI community. Through tools such as IATI Studio, the OpenAid-IATI-Parser (OIPA) and IATI.Cloud, Siem helped organisations transform complex development data into practical, accessible information. In doing so, he expanded what was possible within the IATI ecosystem and helped shape the wider community's thinking on the use of technology.
Over the course of his career, Siem worked with a range of governments and international organisations to strengthen their transparency efforts, including the UK's Foreign, Commonwealth & Development Office (FCDO) and the Global Fund.
Siem Vaessen (second from right), representative of the Technical seat on the IATI Governing Board, at the Board meeting in Vienna, June 2026
Siem cared deeply about IATI, its community and its future. He dedicated countless hours to the initiative, contributing to Technical Advisory Group and community meetings, presenting best practice, and contributing to IATI Standard upgrades. In 2024, he was elected as the Technical seat representative on the IATI Governing Board for the 2024–2026 term.
He brought deep technical knowledge and thoughtful judgement to the Board. He was never afraid to ask difficult questions, challenge assumptions or immerse himself in the detail when it mattered most.
Beyond his professional achievements, Siem was a valued colleague, collaborator and friend.
Much of his work took place behind the scenes, but its impact was felt across the global IATI community. Many organisations have published, explored and used IATI data through the tools and technologies he helped create, and his legacy will continue through the people and systems he inspired.
Our deepest condolences go to Siem's family and his colleagues at Zimmerman during this difficult time.
Leaving a message of condolence
If you would like to share a tribute or message of condolence, you are welcome to do so on our community forum, IATI Connect.