The phrase "leading from behind" is borrowed from group analytic theory, an important branch of group psychology. For some, the phrase may be pejorative: an effective leader is normally in front of group members, not taking a position behind them. However, for large online Communities of Practice, leading from behind and trusting the group is an important strategy. This chapter focuses on how a leader develops the capacity to trust the group. Recognizing that groups of people are powerful and creative organisms that can be trusted is difficult for a leader. For Freud, who thought of groups as unthinking, primitive mobs and for modern managers, who are taught the value of using teams with specific objectives and limited life spans, the idea of unstructured, dispersed collections of people making decisions or taking action is an anathema. Learning to trust the knowledge of a large group takes training, practice and courage. We ground our conclusions in an empirical analysis of the leadership of one large online Community of Practice. Using archives of discussions among community members, we develop leadership principles that support the "leading from behind" approach. We use these data to suggest how managers can lead online communities to form the trusting relationships that are essential for effective knowledge sharing and innovation.