The technological innovation systems (TIS) literature and the strand of system building studies explain the role of networks in the strategic creation of favorable institutional conditions for sustainability transitions. To better understand collective system building, it is important to delve into the formation of networks composed of diverse organizations, including firms, as well as government and civil society organizations, such as alliances. In this paper, we propose an analytical framework for the study of factors that influence the formation of alliances, as well as the contribution of alliances to system building. To illustrate our analytical framework empirically, we conduct a case study of the Green Protein Alliance (GPA), a distinctive example of an alliance network aiming to promote the transition to plant-based diets in the Netherlands. The results highlight the importance of organizational motives, organizational resources, and relationships for the formation of alliances. These factors also influence the type and course of system building strategies, as well as the creation of system-level resources. Moreover, we argue that alliances between diverse types of organizations can provide opportunities to accelerate transitions by promoting the adoption of potentially beneficial innovations and sustainable consumption.