AbstractDigitalization is playing an important role in the emerging practice of Building-to-Grid (B2G). However, the majority of the literature only covers either the grid side, the demand side, or the technical aspect of B2G integration, and an overview of the digitalization in B2G and the involved stakeholders is missing. To fill these gaps, this paper proposes a definition of the B2G ecosystem and provides an overview of the digitalization of the B2G ecosystem with six aspects (B2G goals and themes, B2G technologies, B2G data sources and data management, and B2G related stakeholders). This paper also discusses five emerging technologies (5G, IoT, big data, artificial intelligence, and blockchain) and three main challenges of the applications of the emerging technologies in the B2G ecosystem (Security and privacy, interoperability and scalability, coordination, and dysfunctional emerging behaviors of collective intelligent controls). Lastly, this paper recommends future research directions in the Building-to-Grid ecosystem (especially ecosystem modeling and simulation), B2G’s role in smart cities, sustainability, resilience, and harmonization of the B2G ecosystem, and other emerging technologies in B2G.