PROJECT APPROACH The C-STACS project will bring citizen science approaches to mental health. CHALLENGE: CITIZEN SCIENCE IS NEW TO MENTAL HEALTH Citizen science approaches have not yet been used at scale in mental health. We reviewed citizen science platforms. UK-based Zooniverse has no active, paused or finished mental health-related projects. EU-based eu-citizen.science has one project (CoAct) which includes mental health as one of several global concerns being addressed. USA-based scistarter.org has one project (neureka) using gamification for dementia research. The citizen science projects related to health which do exist typically involve biological and physical health research (e.g. www.cellslider.net, 100forParkinsons App, Colony B App). A more general internet search for citizen science and mental health identifies a few specific studies, e.g. the Emotional Brain Study addressing neuropsychological aspects of mental health and the MH2K Oldham project involving a youth-led approach to mental health, both in the UK, and the Games X Mental Health study exploring interactions between people with mental health problems and their informal carers in Spain. Two citizen science projects have taken place in the UK in relation to the overlapping area of wellbeing: the Secrets of Happiness study which ran in 2018 and has not published results, and the Great British Wellbeing Survey running in 2020. In relation to new projects, no mental health studies were funded as a UKRI Citizen Science Exploration Grant. Turning to publications, again very little has been written about citizen science and mental health. The 2020 ECSA report on characteristics of citizen science provides a useful foundation but no mental health specific guidance. A 2020 overview report by RAND Europe on emerging developments in citizen science identified the potential for new types of health research, such as personal health tracking / n-of-1 studies, but with no mention of mental health. A 2018 paper (https://doi.org/10.1093/heapro/daw086) on citizen science applied to public health provides a useful typology of project characteristics: Aims (investigation, education, collective good, action), Approaches (extreme, participatory science, distributed intelligence, crowd sourcing) and Size (local, mass). Our project will be mass participatory science, with aims of collective good (WP2) and action (WP3). Other papers have used citizen science in cognate areas, e.g. urban stress (https://doi.org/10.1057/s41599-020-0460-1). A 2019 position paper locates citizen science in relation to health research (https://doi.org/10.1080/15265161.2019.1619859), noting the connection with participatory action research and community-based participatory research traditions, and identifies community-driven 'n-of-we' studies which align with the current proposal. The paper also discusses general ethical issues of citizen science in health research, including consent, participation risks/benefits, data ownership and attribution, which have informed the WP1 focus on these issues. Finally, a search of the 'Citizen Science' journal identified one paper (https://doi.org/10.5334/cstp.184) discussing patient partners in health research. WORK PACKAGES (WPs) WP1 develops a theoretical foundation for citizen science in mental health, and prepares for WP2 and WP3. Mental health has a very established tradition of public involvement, coproduction and peer-led research, and mapping between mental health and citizen science constructs will be a key project output. WPs 2 and 3 address specific challenges outlined in the case for support, using Zooniverse as an online platform to support mass participation. WP2 creates new knowledge about supporting mental health recovery and WP3 about how people living with mental health problems actually look after themselves. WP4 mobilises the findings from WPs 1 to 3 to maximise impact.