The Millennium Cohort Study (MCS) follows the lives of around 19,000 children born in the UK in September 2000 - January 2002. A renewed interest in child wellbeing in the late 1990s in the UK led to a new and distinctive child cohort study, after a gap of 30 years. The MCS was designed as a multidisciplinary study that could capture the influence of early family context on child development and outcomes throughout childhood, into adolescence and subsequently through adulthood. Today the MCS covers a diversity of topics including, but not limited to, the cohort members’ health and physical growth, cognitive and socio-emotional development, schooling and leisure, identity and attitudes, relationships and risky behaviours; alongside family context, parenting, and family socio-economic circumstances. It is specially designed to help improve our understanding of smaller groups in the UK — namely those living in disadvantage, ethnic minorities, and families from Scotland, Wales and Northern Ireland. Evidence from the MCS has impacted policy and practice, for example by demonstrating the benefits of breastfeeding on children’s health and cognitive development, the effects of poverty on cognitive development, and contributing to the national evaluation of Sure Start. Study website: www.cls.ucl.ac.uk/cls-studies/millennium-cohort-study/