DISCLAIMER: The present Project Deliverable has been submitted to the European Commission for review. The information and views set out in this report are those of the author(s) and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained therein. CoAct proposes a new approach to face social global concerns with Research and Innovation Actions (R&I Actions) related to mental health care, youth employment, environmental justice and gender equality by engaging citizens as co-researchers. Our approach represents a new understanding of Citizen Social Science (CSS), understood here as participatory research co-designed and directly driven by citizen groups sharing a social concern. The overall objective of CoAct is to develop and demonstrate the scientific relevance and social impact of CSS, which is to date an underexplored area of Citizen Science (CS). This document constitutes Deliverable 2.1 ‘Report on State of the Art of Citizen Social Science’ of Work Package 2 (WP2), which is dedicated to the CSS foundations. It provides a starting point towards a common framework and a common arena to better elaborate the various characteristics of CSS. CoAct wants to contribute to the debate by catalysing the discussion and enlarging the CSS community. This effort is unprecedented in the CS and the Social Science worlds and it is expected to later result into new open materials (for citizens, policy makers, NGOs and academics) and new transdisciplinary methodologies to widen the impact of CS. This report discusses CSS as a component of CS, with its main characteristics that citizens act as co-researchers conducting research on social issues with the aim of achieving transformative and sustainable impact with the research. CSS is however emerging from at three streams: from the broader spectrum of a CS community; from a participatory research background in the social sciences and humanities and moreover directly from Citizen Organizations (CSOs), who initiate and lead research. Thus, the report assembles a detailed review of a vast variety of methodologies, methods, approaches and practices to be applied to R&I Actions and with the aim of delineating a theoretical and methodological framework for CSS. As a result, foundational aspects, challenges and critical topics for further discussion are identified.