Five thousand years ago, the hemlock collapsed as a result of abrupt global climate change. Now this iconic tree faces extinction once again because of an invasive insect, the hemlock woolly adelgid. Drawing from a century of studies at Harvard University's Harvard Forest, one of the most well-regarded long-term ecological research programs in North America, the authors explore what hemlock's modern decline can tell us about the challenges facing nature and society in an era of habitat changes and fragmentation, as well as global change.