The aim of TRANSIT (Transformative Social Innovation Theory) is to build a theory of social innovation that is useful not only to academics, but also to policy makers, social entrepreneurs, and other stakeholders. The starting point for TRANSIT is the need to understand transformative social innovations: social innovations that contribute to systemic changes that address urgent societal challenges. TRANSIT unpacks the relation between social innovation and systemic change in the context of a rapidly changing world that faces ‘game changing’ developments (e.g. financial crisis, climate change or the ICT-revolution). TRANSIT will explore constituent links in the causal chain between social innovation and systemic change. The main research question is: How and under what conditions do social innovations lead to systemic change, and how are actors (dis)empowered in transformative social innovation processes? TRANSIT will develop a new theory of transformative social innovation, drawing upon a range of existing theoretical and methodological approaches to innovation and social change, and using a systems innovation and sustainability transition research framework as a starting point. Empirically, TRANSIT takes an embedded case-study approach to conduct a multi-leveled, cross-national comparative analysis of social innovation projects and networks across Europe and Latin America, combining in-depth case-study analysis with quantitative meta-analysis. The new theory of transformative social innovation is thus both grounded in in-depth case-studies as well as tested and generalised in a cross-national data-base. The research concept of TRANSIT is to create an iterative interplay between: empirical research on social innovation; the development of a new empirically-grounded theory of transformative social innovation; and transdisciplinary translation to capacity building tools to be co-developed with policy-makers, civil society organisations and social entrepreneurs.