Participant engagement in cohort studies has been argued to be a key factor in building trust in the cohort and facilitating long-term retention. Despite the acknowledged benefits of engaging participants in such studies, little research has been conducted to understand what this engagement looks like and how it should be conducted. The proposed study seeks to fill this knowledge gap. Following on from CO’s doctoral study that explored participants’ experiences and perception of their engagement with ALSPAC, the proposed study will: (1) review ALSPAC’s engagement and identify key messages present in their communication with participants; (2) conduct a qualitative study into (ALSPAC and other) cohort personnel’s experiences and perceptions of participant engagement; and (3) combine insights from participants’ perspectives (generated from CO’s previous doctoral research) with insights from the ALSPAC material and the cohort personnel perspectives to formulate a theoretical framework for participant engagement that will explain how and why certain strategies are more or less successful, more or less (un)ethical, and more or less (un)appealing to participants and researchers. In so-doing, this study will produce results that meet CLOSER’s objective to build skills in the management of cohort studies.