As healthcare systems face enormous challenges, sustainability is seen as a crucial requirement for making them fit for the future. However, there is no consensus with regardto either the definition of the term or the factors that characterize a "sustainable healthcare system". Therefore, the aim of this article is twofold. First, it gives examples of the existing literature about sustainable healthcare systems and analyzes this literature with regard to its understanding of sustainability and the strengths and weaknesses of the different approaches. The article then identifies crucial factors for sustainable healthcare systems, and the result, a conceptual framework consisting of five distinct and interacting factors, can be seen as a starting point for further research.