Heads of subject departments in New Zealand secondary schools generally have recognised expertise in teaching their subject and this expertise together with a mixture of leadership and organisational skills are recognised in their selection for the position of Head of Department (HOD). They seldom have formal training in leading staff. Single day training in job skills for new HODs is regularly offered at local education centres but few HODs undertake extended tertiary training in leadership skills. This case study records the experiences of one secondary Head of Department as she does a literature review of coaching and mentoring principles and applies a selection of these principles to her work with her staff. The initial focus of the study was to use coaching and mentoring methods to lead the department in the implementation of differentiation; varying teaching to cater for the diversity of learners. The scope of the study broadened to encompass all coaching and mentoring interactions with department staff. Both coaches and mentors work with individuals to improve performance in an area. The vision of mentoring moves considerably beyond emotional support and provision of sample lessons to developing staff in best teaching practice and developing the teacher's ability to problem solve for themselves. Coaching is described as the art of identifying the coachee's strengths and using this information to choose an effective approach in a given situation. The case study is based on the journal records kept of selected routine interactions with department staff. The journal recorded examples which showed common patterns of interaction, and interactions influenced by the coaching/mentoring literature. A second data source was questionnaire responses from staff who left over the course of the year. Major themes that emerged in the data analysis were an increased awareness of the importance of learning needs when working with individual staff, and working strategically. The Myers-Briggs description of personality preferences was used as a neutral framework for describing personality. Recognising the learning needs that arise from those descriptions gave a framework and guidelines for purposeful coaching / mentoring interactions with staff. The concept of learning preferences brought clarity to individual staff responses to different situations; instead of being idiosyncratic and unpredictable, they can be explained as due to the individual combinations of learning preferences in each staff member. There were a variety of competing demands from outside the department and from individual staff priorities. Over the course of this study the HOD did extensive reflection on her selection of differentiation to cater for student diversity, on using a coaching/mentoring approach for this implementation, and on how to work with experienced staff who may have limited openness to change. The reflection increased her clarity about both the direction and the approaches, and this helped her maintain these in the face of completing demands. There are few studies of New Zealand school middle-managers using coaching and/or mentoring. This case study contributes an example to show other middle-managers what can be done in one year in one specific context; using coaching/mentoring to implement differentiation in a department.