This review sets the context for local Citizen Science (CS) labs to be established in Athens, Berlin, Flanders, Plovdiv and Sofia, by examining a selection of projects from the national CS landscapes. In total, the research team mapped 100 projects across the four countries, collecting information, by means of desktop research, on their type, scale, engagement approach, data collection tools, and impact. Insights gleaned from the review are presented in the form of critical analysis of the identified initiatives and are distilled into a set of recommendations intended to help COMPAIR pilots deliver successful CS campaigns. In addition, this deliverable fills gaps in existing literature by providing a nuanced perspective on the state of play of CS in Bulgaria, Belgium, Germany and Greece, and it advances the state of the art of CS research by proposing a new typology of CS regimes. ...