The analysis of the cognitive role played by external representations – particularly within the distributed cognition (DC) framework – has focused on the use of such representations in cognitive tasks. In this paper, we argue that the processes of building such representations require close attention as well, especially when extending the DC framework to ill-structured domains such as scientific laboratories, where building novel representations is crucial for making discoveries. Based on an ethnographic study of the building of computational models in a systems biology laboratory, we examine the complex cognitive roles played by the external representations built by the lab (pathway diagrams and models), and the building process itself.