For years, evidence of a birthdate effect has stared out of qualifications data for the United Kingdom; summer-born children appear to be strongly disadvantaged. Whilst those responsible for working on this data have tried to bring public attention to this issue, it has been neglected by agencies central to education and training policy. Researchers at Cambridge Assessment have had a long interest in the birthdate effect because it is so readily observable in the assessment data with which they have worked. More recently, Cambridge Assessment decided to review the issue with the intention to advance the understanding of the extent and causes of the birthdate effect in the English education system. Although the review focuses on understanding the birthdate effect in England, it uses international comparisons as one means of throwing light on key factors. This article outlines the findings of the review.