This paper explores the nature of a critical friendship between two education advisers within a nation-wide New Zealand Ministry of Education (MOE, 2006-2008) research and development project. Over 18 months the relationship developed through evolving phases. This paper identifies and discusses the factors, circumstances, conditions and analytical tools that contributed to these changes; reviews appropriate literature; and responds to five questions that develop further understandings and insights into critical friendship. Findings from this government initiative (extracted from field notes, transcribed conversations and interviews) provide evidence to inform a model for the developmental phases of critical friendship that has international application within educational professional development. (Contains 1 figure and 3 tables.)