Research has made seminal contributions to describing the size and scope of the Third Sector, including volunteering as an essential component. However, most of the research has focused on economic benefits (revenues, employment etc.). We highly value these efforts, but posit that the core contribution and main impact of the sector on socio-economic development lies in the creation of social innovation.Our claim is that the Third Sector is better equipped to foster social innovation than the market or the public sector. We will build a set of testable hypotheses that relate to its key characteristics, e.g.: (1) strong value sets; (2) persistent multi-stakeholder constellations; (3) the mobilisation of multiple resources.Against the update of structural data, we aim at testing these hypotheses on the qualitative impacts of the Third Sector in terms of capital building (e. g., social networks, cultural values or political participation) and their direct link to social innovation. We set out to investigate organizations with a special emphasis on volunteering at the micro level, which serves as a bond from and into society. The analysis will include a screening of framework policy conditions and discourses (citizens/media) on the roles and functions of the sector. This strategy will be executed in case studies against major social innovation trends of the last years.The fields we aim to study across 9 countries are: (1) arts & culture; (2) social services & health; (3) environmental sustainability & consumer protection in finance; (4) work integration & community development; all of which are of vital importance face to current societal challenges.By doing so we will not only redefine perspectives on the sector to stress its main impact as driver of social innovation, but also relate to recent EU research (SELUSI, TEPSIE, INNOSERV, WILCO or CINEFOGO) to lift the investigation of social innovation to systemic level highlighting comprehensive innovation ecosystems.