This paper aims to develop a conceptual model and a research framework for study of the economic, social and environmental sustainability of digital libraries. The major factors that are related to the economic, social and environmental sustainability of digital libraries have been identified. Relevant research in digital information systems and services in general, and digital libraries in particular, have been discussed to illustrate different issues and challenges associated with each of the three forms of sustainability. Based on the discussions of relevant research that have implications on sustainability of information systems and services, the paper proposes a conceptual model and a theoretical research framework for study of the sustainability of digital libraries. It shows that the sustainable business models to support digital libraries should also support equitable access supported by specific design and usability guidelines that facilitate easier, better and cheaper access; support the personal, institutional and social culture of users; and at the same time conform with the policy and regulatory frameworks of the respective regions, countries and institutions. It is also shown that measures taken to improve the economic and social sustainability should also support the environmental sustainability guidelines, i.e. reduce the overall environmental impact of digital libraries. It is argued that the various factors affecting the different sustainability issues of digital libraries need to be studied together to build digital libraries that are economically, socially and environmentally sustainable.