In Round 7, Young Lives collected hair samples from study participants for the first time, with the aim of measuring cortisol levels, a correlate of chronic stress. This technical note summarises the scientific basis for using hair cortisol as an indicator of chronic stress, outlines the sample collection protocols, and describes how Young Lives country teams in Ethiopia, India and Peru implemented these protocols in practice. It offers a useful tool to assess Young Lives’ process of hair sample collection, makes recommendations for future rounds, and provides other researchers with valuable information to produce large-scale cortisol data in international cohort studies.