Critical friendship is a flexible form of assistance for development and research, and one that had a key role in the international Carpe Vitam Leadership for Learning project. Whilst conforming to common features and engaging in similar activities there were some differences in the way critical friendship was enacted across the eight sites. The work of the critical friends contributed to the development of the leadership for learning principles, which in turn provide a framework for considering critical friendship and deepening our understanding of its nature. Dialogue, a focus on learning, and concern with the conditions for learning are all found to be core features of critical friendship, and were themselves subjects of the critical friends' work. (Contains 1 note.)