This doctoral thesis aims to trace the impacts of campaigns carried out by coalitions of social movement organisations in the transnational arena of the EU. In order to accomplish this task, an original approach to process tracing is adopted using methods used in social movement studies. The internal aspects of campaigns are investigated using a dynamic, cross-time and multi-level, frame analysis, while the contexts of the campaigns are analysed through political and discursive opportunity approaches adapted to the peculiarities of the EU arena. Four case studies, including two campaigns concerned with environmental / public health policy (GMOs and coexistence, and the REACH legislation) and two concerned with broadly defined social policy (the mid-term review of the Lisbon agenda and the Services directive), make up the empirical part of the study. Drawing on documentary evidence as well as semi-structured interviews with staff members from the core SMOs involved in each campaign at the Brussels level, the processes leading to access, agenda, or policy outcomes (or indeed non-outcomes) are traced using the analytical methods mentioned above. These processes provide the basis for preliminary conclusions on the nature of campaigning in the EU. Elite allies are found to be important in securing desired outcomes in campaigns, as are solid, previously agreed shared frames between coalition organisations. The cases also show that the EU is not an arena where conventional tactics (i.e. lobbying) are always enough – indeed the ability to campaign effectively at multiple levels using appropriate tactics is identified as a major factor in campaigns that saw positive outcomes. This finding challenges the idea that the EU arena is unsuitable to protest actions (e.g. Marks and McAdam 1996). Finally, the study uncovers the beginnings of a divide between ‘technical’ and ‘political’ campaigns in the EU. Stemming from the finding that national contexts still provided the opportunities or threats that appeared most important in campaign outcomes, the cases showed that where campaigns were more ‘political’ - in that they were more ideologically charged - groups were more likely to be able to mobilise grassroots members and secure their desired outcomes. In more ‘technical’ cases, where the European Commission played a greater role, mobilisation efforts were subdued as groups sunk their resources in long cycles of consultation and knowledge production geared to the needs of the Commission.