There has been research on grouping pupils by ability for most of the twentieth century since Whipple carried out a study of the effects of special class placement on a group of high aptitude 5th and 6th graders in the USA in 1919. Since then hundreds of studies have been undertaken and there have been many literature reviews and syntheses of research findings. Historically, streaming was the dominant form of grouping adopted in the UK. This article provides a brief historical perspective on structured ability grouping, a summary of recent research on streaming and setting amongst seven-year-olds from the Millennium Cohort Study, and considers some of the implications of what appears to be an increase in structured ability grouping in the primary school.