Companies that wish to launch innovative sustainability technologies can collaborate in strategic networks of actors from industry, government and research institutes to pro-actively build a business ecosystem around their new technology. This is called collective system building. In this paper, we examine how to effectively manage networks for collective system building. Based on a review of the literature, we identify the key factors of effective network management and we propose a conceptual framework for network management at the network level. Subsequently, we conduct a multiple-case study in the Dutch smart grid sector to examine how these key factors are implemented by system-building networks. We find differences with the existing network management literature regarding network composition, network management structure, governance modes, decision-making processes, project management, the free-rider problem and trust-building mechanisms. Our study contributes to a better understanding of effective management of system-building networks, which in turn can lead to greater success in establishing new business fields. We contribute to the literature on strategic business networks, specifically on emerging business networks building new business fields.