This article introduces this special issue on the substance of EU democracy promotion. It explains the research question that is central to this special issue: What is the democratic substance that the EU furthers in third countries? First, we provide a review of the literature, arguing that existing studies have mainly focused on the impact and strategies of EU democracy promotion without sufficiently analysing the very substance it furthers in third countries and regions. While academic research has made a rough distinction between a broad and a limited notion of democracy promotion, scholars have not yet systematically and comparatively analysed this topic. Second, we develop a conceptual framework in order to 'map' the substance of EU democracy promotion. Starting from a liberal conception of democracy and based on an adaptation of the model of 'embedded democracy', different components, types and agendas of democracy promotion are identified. Third, we formulate a number of expectations on the substance of democracy advanced by the EU, focusing in particular on the distinction between a 'one-size-fits-all' and a differentiation scenario. This article ends with a summary of the different contributions to the special issue.