Citizen science describes research activities (e.g., data collection) that are conducted by people who are not professional scientists. Thanks to the internet, citizen science has gained a new dimension: digital technologies offer possibilities to reach out to volunteer researchers, to pool together efforts and to make the results visible. Moreover, the collective power of the crowd provides scientists with data that they could not have collected on their own. Integrating citizens into research projects is apparently a valuable strategy for scientists. Motivating volunteers to invest time in citizen science projects appears challenging.