This paper analyzes the award-winning e-participation initiative of the city council of Madrid, Decide Madrid, to identify the critical success factors and the main barriers that are conditioning its performance. An exploratory case study is used as a research technique, including desk research and semi-structured interviews. The analysis distinguishes contextual, organizational and individual level factors; it considers whether the factors or barriers are more related to the information and communication technology (ICT) component, public sector context or democratic participation; it also differentiates among the different stages of the development of the initiative. Results show that individual and organizational factors related to the public sector context and democratic participation are the most relevant success factors. The high expectations of citizens explain the high levels of participation in the initial stages of Decide Madrid. However, the lack of transparency and poor functioning of some of its participatory activities (organizational factors related to the ICT and democratic dimensions) are negatively affecting its performance. The software created for this platform, Consul, has been adopted or it is in the process of being implemented in more than 100 institutions in 33 countries. Therefore, the findings of this research can potentially be useful to improve the performance and sustainability of e-participation platforms worldwide.