It is with the greatest sadness that we pay tribute to Prof Margaret Kelaher who passed away suddenly in March 2021. Margaret was a cherished and integral member of the C4 team and throughout her career made an enormous contribution to health policy, public health, and the field of implementation science. As a C4 Chief Investigator, Margaret’s expertise in implementation and evaluation contributed greatly to the work of the CRE. Margaret was an extraordinary colleague from whom we still had much to learn. We fondly remember her passionate commitment to equity, her infectious laugh, her collegiality, and the incredible mentorship she provided to many members of the team. Vale Margaret.