The ‘Healthy School Lunch’ project (PPS Een gezonde schoollunch - TKI AF-16098) aimed to study the feasibility and impact of offering a healthy school lunch in Dutch primary schools. In the first phase of the project we studied the support for a healthy school lunch among a wide range of stakeholders and we explored what they thought a healthy school lunch should look like. The next phase aimed to study the effects of a healthy school lunch on dietary intake and cognitive performance. In order to investigate the effects of providing a healthy school lunch on the dietary intake of children during lunch, a longitudinal intervention study with three schools was designed, where a healthy school lunch was offered for six months. Due to various financial and methodological reasons that were not fully considered when starting this project, it was not possible to combine the dietary intake and cognition study in the school lunch intervention. Instead, we explored the possibilities and requirements to perform a solid experimental cross-over study on the effects of a healthy school lunch on cognitive performance within the Healthy School Lunch Project. This process is described in the current report. We started with a review of the literature on the effects of a healthy school lunch on cognitive performance of children. Studies on the immediate and transient effects of a healthy school lunch versus skipping lunch showed, on a variety of cognitive measures, small and inconsistent effects on alertness and working memory of children. Studies on the long-term effects of a healthy school lunch versus habitual lunch showed small improvements in concentration and language processing ability of children. Given the restriction that long term effects of a school lunch on cognitive performance could not be examined within the scope of this project, the focus shifted to understanding immediate effects. A well-designed study to capture immediate effects of a healthy school lunch on cognitive performance of children within our project would require a strictly controlled cross-over design in a school setting. The intervention should consist of an ad libitum buffet-style healthy school lunch, compared with a control condition in which children eat ad libitum from a provided lunch comparable to the common relatively unhealthy packed school lunch of Dutch children. Finding an adequate measure for cognitive performance is hampered by the great variation of previously used measures across studies. These ranged from relatively indirect measures of concentration and disengagement to standardized computerized tests assessing specific cognitive domains such as alertness and higher-level executive functions. Hence, a comprehensive battery of tests would be advisable to explore various potential effects. Power calculations would be needed to determine the sample size of such a study, but it is clear that large numbers of children would be needed, given the small expected effects and methodological challenges. As more than one participating school would be needed, multilevel statistical models would be required to handle grouped and individual children’s data. More research on the effects of a healthy school lunch on cognition in primary school children would be very useful. However, a well-designed study that would provide convincing evidence of the effects of a healthy school lunch on cognitive performance in primary school children, would require a high cost set-up that places a very high burden on both the children and the schools. Therefore, we decided that it is not realistic to perform this study within the Healthy School Lunch project. In this report we would like to share our findings, considerations and recommendations to researchers of future studies on the effect of healthy school lunches on cognitive performance in children.