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Citizen science projects involve the public with scientists in collaborative research. In-formation and communication technologies for citizen science can enable massive virtual collaborations based on voluntary contributions by diverse participants. As the popularity of citizen science increases, scientists need a more thorough understanding of how project design and implementation decisions affect scientific outcomes. Applying a comparative case study methodology, the study investigated project orga-nizers ’ perspectives and experiences in Mountain Watch, the Great Sunflower Project, and eBird, three observation-based ecological citizen science projects in different scientific do-mains. Five themes are highlighted in the findings: the influence of project design ap-proaches that favor science versus lifestyle; project design and organizing implications of engaging communities of practice; relationships between physical environment, technologies, participant experiences, and data quality; the constraints and affordances of information and communication technologies; and the relationship of resources and sustainability to institu-tions and scale of participation. This research contributes an empirically-grounded theoretical model of citizen scienc