BackgroundDespite insufficient evidence of effectiveness, mentoring programmes are commonplace in various contexts in the UK. Mentoring has potential to influence health and educational outcomes, so it is of public health interest to obtain a better understanding of the types of programmes currently available to aid evaluation and service commissioning. The aim of this study was to develop a typology of currently active mentoring programmes that provide formal mentoring for young people in UK secondary schools. MethodsEight websites were searched to retrieve details of mentoring organisations. Organisations were purposively selected using maximum variation sampling, and programme providers were invited to take part in semi-structured telephone interviews. The typology was consulted in a three-step process—interviews with purposefully selected experts in mentoring, providers' written feedback, and follow-up interviews with experts. Interviews were audiotaped, transcribed, and analysed with thematic and framework analysis. All participants provided written consent, and ethics approval was granted by the University of Bristol. Findings23 of 29 invited providers and all five invited experts took part in the study. Providers described 28 diverse formal mentoring programmes, and participants differed in the way they conceptualised mentoring. The final typology differentiated mentoring programmes by three overarching categories: type of mentor (older student, adult volunteer, school staff, paid adult), programme setting (school, community, online), and programmes' overall aim. On the basis of their overall aim, programmes were allocated to two broad groupings of personal and developmental and academic and employability mentoring programmes. 12 mentoring models were identified on the basis of different combinations of subcategories. InterpretationAlthough mentoring programmes are heterogeneous, it is possible to identify key characteristics and to categorise existing programmes into one of 12 mentoring models. Use of semi-structured telephone interviews allowed for a thorough investigation of differences between mentoring programmes that was grounded in participants' accounts of their programmes. Further study is needed to test the typology's generalisability in the UK. Such a typology can help us to understand what is being delivered, for whom, and how, which is a necessary precursor to any public health evaluation. FundingThe work was undertaken with the support of the Centre for the Development and Evaluation of Complex Interventions for Public Health Improvement (DECIPHer), a UKCRC Public Health Research Centre of Excellence. Joint funding (MR/KO232331/1) from the British Heart Foundation, Cancer Research UK, Economic and Social Research Council, Medical Research Council, the Welsh Government, and the Wellcome Trust, under the auspices of the UK Clinical Research Collaboration, is gratefully acknowledged.