Citizen social science is an emerging, multi-sited, socially structured practice for social research. As an innovative form of coproduction and participatory research, it involves engaging citizens in conducting social research. To date, when citizen social science has been theorised, the focus has usually only been on its potential as method: as a form of crowd-sourced data collection, and in a context of increasing technological advancements in, and possibilities for, data gathering. Just like citizen science, which involves the public in large-scale collective volunteer science projects, citizen social science presents both a challenge and an opportunity. It can challenge the ways in which social science research is undertaken, raising questions around who can collect data, who can analyse it, and how it can be used. The possibilities of citizen social science bring into focus issues of data quality, diverging motivations, claims of expertise and skills, and the relationship between data collection, analytical frameworks, and social realities. There is the potential for the flattening of hierarchies in the social research process as knowledge is made together. The thesis seeks to understand how citizen social science works in practice. It reports on three social science probes using citizen social science-based approaches: i) a secondary analysis of Mass Observation Archive data, ii) a study involving citizens reporting observations of empty houses, and iii) a community based history project about perceptions of the changing nature of a local area. The research sets out three key facets of citizen social science: the challenges for research design and execution, the ethical issues raised, and the potential for data use. The thesis elaborates on them as new evidence of the practices, processes and challenges of citizen social science. The findings suggest that citizen social science-based methods can be disruptively transformative: for the individual participant in terms of how they can become engaged with the issues raised by the research; for the discipline, in the way that it allows seeming non-experts to be involved in the analysis of data, as well as its generation; and, moreover policy makers are beginning to take such data seriously. Overall, the research highlights how citizen social science can be an exercise in the collective sociological imagination and collective experimentation. It presents an opportunity for the transformation of social science research, beyond just an instrumental methodological innovation, but there are challenges and limitations.