In the past 100 years, the U. S. National Park Service has played a key role in nature conservation. Yet despite efforts to protect and conserve nature in the U.S. and around the world, human activities are increasingly leading to the deterioration of habitats and the loss of species. How can natural systems be protected both in nature reserves and elsewhere? How can economic demands and conservation goals be reconciled? And what are the main threats that habitats and species face today? This curated collection of Science articles explores some of these issues.