Critical friendship is a versatile form of external support for school colleagues engaged in leadership activities, and one that is subject to increasing professional and political interest. This article focuses upon the contribution of critical friends supporting leadership and school improvement in a range of contexts, including an international research project, school self-evaluation, and networking. It draws upon activities of "Leadership for Learning: the Cambridge Network" to explore the role and functioning of critical friends by addressing 10 questions. The article concludes by summarizing the insights that can be gleaned from the examples of critical friendship in operation, and suggests that as the use of critical friends to support leadership in schools becomes more of the norm there is a need both to draw upon what is already known about critical friendship, and to extend our understanding.