Abstract

: The Millennium Cohort Study (MCS) is a major UK-wide ongoing longitudinal birth cohort study, which has had eight major surveys to date, from 9 months to age 23 years. The present review reflects on the design and development over the study's first 18 years. The initial sample design is stratified and clustered, and a strong focus was placed on keeping engagement in the study and response rates high. Some of the study's key innovations included the objective measurement of human development from as young as age 3, and the centrality of children's voices through its questionnaire design. MCS faced some unique ethical considerations, as one of the first major longitudinal studies to directly survey children from a young age, and was at the forefront in developing ethical best practice in surveying children and young people. MCS embedded early research findings into its remit by producing research reports and briefings after each survey, which not only helped highlight the potential of the data to the wider research community, but also played an important surveillance type role in evidencing major issues emerging for this generation. Finally, as one example of its policy impact, MCS contributed to understanding young people's mental health and wellbeing by highlighting the scale of difficulties facing this generation and widely disseminating the findings.