The aim of this PhD research project is to investigate to what extent and how social media are reshaping sourcing and agenda-setting practices in mainstream journalism and how this affects the mediated debate on public affairs. To this end, a three-step multi-method research design is proposed, which will generate statistical evidence and in-depth insights into the role of social media in the flow of news in today's networked public sphere. First, in order to examine inter-media agenda-setting relations between old and new media, time series analyses will be done on an integrated large-scale dataset of news output from different Flemish media outlets, including newspapers, TV, radio, online and social media. Next, the study will take a closer look into the ways in which social media, and particularly Twitter, are handled as sources of information in mainstream news stories, and vice versa. This will be done by means of a qualitative content analysis, which will be supplemented with in-depth reconstruction interviews with professional journalists to deepen our understanding of journalists' sourcing practices in the social media age. The research project focuses on three different 'news beats' or 'issue domains' in the journalistic field: political news, economic news and crime and justice news, each reflecting a dominant field of power in society (i.e. politics, business and the judicial field). Through the combination of agenda-setting and gatekeeping theory and the focus on social media, this project will generate unique and original results on the flows of news in today's 24/7, cross-media, networked news ecology.