The social, organizational, and technological complexity of high-risk organizations creates a strong need for coordination. Joint activity in such systems thus relies both on formal coordination and on a set of informal coordinative practices. Coordinated joint activity is documented in this paper from two perspectives: the development of coordinative practices to overcome the limitations of formal coordination, and the use of artifacts for coordinative purposes. The empirical material is provided by a workplace study undertaken according to theoretical conceptions of situated action and cognition. It was undertaken in the context of a design project that aims at improving the tagout process in a high-risk industry. Findings first describe and analyze situations in which formal coordination turned out to be more of a constraint than a resource for effective action. They then illustrate the role played by artifacts in coordinated activity, focusing on the use of a particular artifact, the tagout tag, in different situations.