Summary Young Lives is an international longitudinal study investigating the changing nature of childhood poverty in four low-income countries [Ethiopia, India (Andhra Pradesh), Peru and Vietnam] over a 15-year period. In each country, the cohort is comprised of ~2000 children aged between 6 and 18 months and up to 1000 children aged between 7 and 8 years, recruited in 2002 and sampled from 20 sentinel sites. The first survey data collection from primary caregivers and older children took place in 2002, the second in 2006–07 and the third in 2009–10. Data on the community contexts were collected to complement the household surveys. To elaborate and extend the quantitative data, longitudinal qualitative research with a subgroup of the children was carried out in 2007, 2008 and 2010–11. Topic areas covered included nutrition, health and well-being, cognitive and physical development, health behaviours and education, as well as the social, demographic and economic status of the household. Survey data from the study are archived in the International Section of the UK Public Data Archive.