Tracing how children transition through schooling institutions and their engagement with wider social processes as they move through later childhood. Finally, the paper discusses policy and programming implications of Young Lives longitudinal research to date. A longitudinal cohort study Despite the scientific and policy potential of longitudinal analysis, cost and capacity mean that cohort data are rare, particularly in developing countries. Young Lives is unique in spanning four countries, two cohorts, diverse locations, and employing a mixed method design. The 'older cohort' was born in 1994/5 and the 'younger cohort' born in 2000/1. Household, caregiver and child surveys have been conducted in 2002, 2006, 2009, 2013/14, with a further round planned for 2016. In this way, field workers have been visiting communities and interviewing families and children at key age points during early childhood, middle childhood, adolescence, and into early adulthood. These surveys track: changing poverty circumstances, risks and opportunities, aspirations for and of children; health, nutritional, educational, and personal histories; children's time use, including domestic responsibilities, employment as well as school attendance; access to key services-health, education and social protection; cognitive, psychosocial, educational and other developmental outcomes. Core household surveys are complemented by school surveys embedded within country specific education systems, curricula, including specially designed achievement tests. Survey and psychometric data is complemented by qualitative longitudinal research with a subsample of approximately 200 young people and their caregivers, nested within the wider Young Lives sample across all four countries.