The internet has radically transformed traditional political mobilisation and participation: participation costs have become extremely low; the need for co-presence evaporated; flexible, horizontal institutional structures replaced conventional organisations; and content can be produced and distributed by everybody easily. Social media content provides direct access to networks and content produced by citizens. It can not only reveal their attitudes towards policy problems, politicians, elections, riots, protests and unrest, but also highlight people’s preferences, willingness to participate and mobilise others. The present project exploits this new type of information aiming to deepen our understanding of citizens’ decision to participate politically. The main research questions are (a) how do social media (re)shape the relationships between citizens and politics (communication), and (b) how do these media affect the willingness to become politically active (mobilisation). The project combines new methodologies and techniques for handling and analysing large-scale social media data in combination with survey data on political behaviour.