With citizen science, volunteers are engaged to help with tracking, analysis, and discovery on scientific projects. Citizen science efforts provide valuable contributions that advance scientific inquiry and knowledge, while engaging and educating volunteers. Examples include enlisting volunteers to help with the labeling of heavenly bodies in astronomical images, identification of birds in the wild, the tracking of neuronal paths through multiple cross-sections in microscopic images, and engaging the public with online games that help molecular biologists find better solutions to protein folding problems. This symposium describes several citizen science projects, focusing on key efforts in astronomy and biology. It includes presentations on the GalaxyZoo and Zooniverse projects, based at Oxford University and the Adler Planetarium of Chicago; work on eBird at Cornell University; and efforts at Microsoft Research to develop computational methods aimed at providing a “science of citizen science,” with the development of methods for the ideal fusing of multiple contributions via Bayesian inference procedures.