Although firms increasingly rely on interfirm collaboration to explore new technological opportunities, few studies have examined the ways in which this collaboration form is governed. In this paper, we attempt to increase our understanding of governing explorative R&D alliances. First, we argue that explorative R&D alliances are likely to face substantial risks of opportunistic behavior and high coordination costs. Second, applying insights from the alliance literature, we identify formal governance mechanisms as effective tools to mitigate the risk of opportunistic behavior as well as coordination costs. However, relying on insights from the new product development literature, we argue that such formal governance mechanisms may hamper exploring new technological opportunities. Third, we propose and develop alternative strategies, facilitating the governance of explorative R&D alliances: 1) relying on embedded relationships in which heterogeneity is maintained and 2) balancing formal and relational governance. To conclude, we emphasize the importance of longitudinal, multi-level research to study the characteristics and dynamics of different governance mechanisms within interfirm collaboration.