Young Lives is an international study of childhood poverty following the lives of 12,000 children in Ethiopia, India, Peru and Vietnam over more than 15 years. Undertaking longitudinal research in low and middle income countries brings a host of challenges often not present in other forms of research. In this series, supported by the ESRC and GCRF, we reflect on our methodological learning, and the lessons we have drawn from our extensive experience in longitudinal research and the many challenges we have faced.