The Resource Teacher Māori service (RTM) has been working to lift the performance of principals and teachers of Māori medium schools for over 25 years as Māori medium education has attempted to achieve the twofold aim of revitalizing the Māori language and increasing the success of Māori students in Aotearoa/New Zealand. While there have been many positive gains over that time, issues still remain in fulfilling these aims. Therefore, an important question that needs to be answered is to what extent have RTM been successful in supporting schools to realize the key aims of Māori medium education, and what issues still remain in the service they provide? The central aim of this research project was to examine how Resource Teachers Māori (RTM) facilitated professional development (PD) in schools, what issues they faced, and how they negotiated them. The study was a multiple case study design of three RTM, principals and teachers and used conversational interviews as the method of data gathering. The research found that the RTM service was viewed positively by schools and had a positive influence on teacher skills and student learning outcomes. Of particular importance was the Māori language and cultural guidance the RTM provided teachers. They were also highly committed to the Kaupapa Māori philosophy and to Māori language revitalization. However, their potential was not always realized. Factors that affected the performance of RTM included having a large number of schools to service and a wide range of teacher needs. The support the RTM received at the national and local levels was also insufficient. An implication of these findings is that the RTM service should receive greater support and guidance, but there should also be a greater degree of accountability that the service the RTM are providing is meeting school needs.