This working paper focuses on secondary analysis, an aspect of research practice that is sometimes assumed to pose few ethical challenges. It draws in particular on the experience of a collaborative research project involving secondary analysis of qualitative data collected as part of an ongoing international longitudinal study, Young Lives, and sets this alongside a wider review of regulatory guidance on research ethics and academic debates. Secondary analysis can take many forms, and bring many benefits. But it is more ethically complex than regulatory frameworks may imply. Whether or not data are publicly archived, ethical considerations have to be addressed, including responsibilities to participants and the original researchers, and the need to achieve a contextual understanding of the data by identifying and countering risks of misinterpretation. The considerations raised here are intended to aid ethical research practice by supporting planning and reflection – for primary researchers who are planning to archive their data, as well as for researchers embarking on a qualitative secondary analysis. Not least, our experience highlights the importance of developing and maintaining trusting relationships between primary and secondary researchers.