The professional development of teachers is a core process in any system which hopes to offer a high quality of education to its population: ‘the best school systems are those that have the best teachers’ (Barber and Mourshed, 2007, p. 7). Whilst there is a wide range of approaches to support professional development, the use of mentoring remains a core process by which professionals are able to support each other to develop their practice (Hudson, 2013). This chapter considers the epistemic and practice-based environment in which mentoring occurs. What are the assumptions we make about both what it means to be a teacher and the process of practice development? To consider these questions we reflect upon the role of the teacher and the processes this involves through the lens of critical processual complexity and the use of the pedagogic black box and pedagogic literacy as useful models to show how a flexible, critical approach to mentoring might be established. We also draw upon case studies from our experiences of working with mentors and beginning teachers.