In recent decades, problems related to the accessibility and sustainability of science have increased, both in terms of the acquisition and dissemination of knowledge and its generation. Policymakers, academics, and, increasingly, citizens themselves have developed various approaches to this issue. Among them, citizen science is distinguished by making possible the generation of scientific knowledge by anyone with an interest in doing so. However, participation alone does not guarantee knowledge generation, which represents an epistemological challenge for citizen science. Simultaneously, economic and socio-institutional difficulties in science governance and maintenance have grown. To solve those problems, several market elements have been introduced, a solution rejected by those who consider science as a public good that states must guarantee. Alternatively, research and work on the commons are growing worldwide, the concept being extended from natural resources to knowledge resources. In this chapter, we propose science as a commons, underlining the essential role of citizen science. Difficulties also apply to citizen science itself, but the increasing development of a multitude of projects based on cooperation favours the conditions required for its sustainability and quality. Our philosophical proposal is based on empirical knowledge about citizen science coupled with socio-economic concepts, according to a sociopolitical epistemology.