Purpose: This paper synthesises international research on effective mentor training, education and development (MTED). Design/methodology/approach: An adaptive theory methodology (Layder, 1998), combining deductive and inductive methods, was deployed in a qualitative meta-synthesis of thematic findings generated in three studies: a systematic review of literature published between January 2010 and July 2020, together with a secondary analysis of studies including evidence on MTED; a subsequent systematic review of literature published between August 2020 and May 2023 and a general inductive analysis (Thomas, 2006) of interviews of leaders of large-scale MTED programmes that had good evidence of impact. Findings: Our meta-synthesis found that effective MTED is evidence-based, refined through ongoing research, tailored both to individual needs and context and includes sustained support. Effective pedagogical approaches in MTED are underpinned by adult learning principles and establish a learning climate that fosters open and trusting relationships. Effective MTED is shaped by the espoused mentoring model or approach, with particular emphasis on understanding, building and sustaining mentoring relationships and incorporating observing, practising, critically reflecting on and receiving feedback on mentoring. Practical implications: The study will be helpful to practitioners designing, reviewing and evaluating MTED programmes, researchers seeking to enhance the sparse MTED evidence base and programme commissioners. Originality/value: The original and significant contribution of this study is the identification of key principles relating to the overall design of -- as well as specific content, pedagogical approaches and supporting resources within -- MTED programmes that have evidenced positive effects on mentors, mentees, mentoring and/or organisations.