In this thesis, I develop and then test an approach to mentoring that after Clutterbuck (1991), I call, Professional Friendship. This is to better understand the role of mentoring more broadly and that of learning mentors in particular. My hypothesis examines the problem of: 'To what extent is professional friendship a core component of successful mentoring relationships?' I begin with an examination of the importance of mentoring generally and the role of the learning mentor specifically, before critically accessing the literature on both to date. I then explain the origin of Professional Friendship and give my own definition, before testing its validity in a study of the role of learning mentors in a large West Midlands Education Service. I use five sources of data: a systematic literature search; a mentee questionnaire; a set of mentoring case studies provided mainly by mentors; in depth interviews with mentors; and my own lived experience. The data shows me that Professional Friendship is a core component of successful mentoring relationships and that may be of broad benefit; certainly it is useful in assessing the role of learning mentors. However, the analysis also leads me to suggest ways in which the construct can be adapted and improved.