This article presents findings from longitudinal ethnographic research of a mega-project alliance. For five years we followed the leadership team of a large Australian Alliance Program made up of a large public and several private organizations, analyzing 'practice' as novel patterns of interaction developed into predictable arrays of activities, changing and transforming while at the same time continuing to be referred to as 'the same'. In this article we focus on three such arrays of activities: authoring boundaries, negotiating competencies and adapting materiality. We suggest that these are essential mechanisms in becoming a practice. While most studies of practice deal with already established practices, the significance of our research is that we develop a notion of practice as it unfolds. In this way we can provide a better account of the constant change inherent in practices.