The teaching profession, at its core, is relational. Professional development of teachers is provided in many ways: one day seminars, professional learning communities, mentoring, etc. However, there has been little attention to the interaction between the teachers involved in the process and what influence this interaction has on their professional development. Using Bruner's (1996) psychocultural perspective, this case study explores the development of a knowledge community of two middle school teachers in a mentoring experience. Through an examination of the developing relationship in context, we examine the ways in which their interactions fostered change for both teachers in their professional practice, personal relationships, and the school community in which they engaged, and identify the interactional dimensions of such change. This study challenges the computational approach to teacher professional development and suggests that more attention to the relationship may foster long- lasting teacher growth.