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.