This session investigates the role of science amateurs for the public understanding and public engagement with science. Insight can be drawn from recent and ongoing research on amateur and citizen scientists from the United States, Canada, Germany, and the United Kingdom regarding the impact of hobbyists conveying science to the public. Specifically, the session will discuss how serious leisure enthusiasts like amateur astronomers or avid birders play a role in sharing their passion and their particular understanding of science with the public. Research on how citizen scientists and their supporting organizations operate indicates that these groups can play a major role in shaping the public understanding of and engagement with science; moreover, the research suggests a model for acquiring science knowledge and understanding that focuses on specific islands of expertise first, rather than first attempting to support general knowledge of science that is easily transferable across domains. Because many citizen science projects span across political borders, particularly those organized via online tools, citizen science is also becoming a mechanism for internationalizing public engagement with science and is serving as a bridge for cross-cultural understanding.