In the past two decades, large-scale assessment schemes, such as PISA, have been set up at an international level as accountability tools for education systems, whose results and conclusions bear relevant repercussions for education systems and policies around the world. While the guidelines for student sampling and exclusion are explained in detail and applied consistently in all countries to ensure comparability, there is some leeway for countries to control the participation of Special Education Needs (SEN) students in the test. This paper discusses the lack of transparency regarding the participation of SEN students in PISA, relying on literature to fill in the gaps that are encountered in the documentation provided by the OECD. It also reviews the difficulties faced both by SEN students and researchers wishing to focus on the results of this particular collective, presenting the main conclusions drawn by researchers on this field. The paper concludes with a reflection on the need for further research and a higher transparency regarding both the participation of SEN students in PISA and the publication and report of their results.