**A *San Francisco Chronicle* Best Book of 2016: “Intelligent and impassioned, *Citizen Scientist* is essential reading for anyone interested in the natural world.”** Award-winning writer Mary Ellen Hannibal has long reported on scientists’ efforts to protect vanishing species, but it was only through citizen science that she found she could take action herself. As she wades into tide pools, spots hawks, and scours mountains, she discovers the power of the heroic volunteers who are helping scientists measure—and even slow—today’s unprecedented mass extinction. Citizen science may be the future of large-scale field research—and our planet’s last, best hope.