Harris, T. B. (WRA, Inc., 2169-G East Francisco Boulevard, San Rafael, CA 94901), N. Rajakaruna (College of the Atlantic, 105 Eden Street, Bar Harbor, ME 04069), S. J. Nelson (Senator George J. Mitchell Center for Environmental and Watershed Research and Department of Plant, Soil and Environmental Sciences, University of Maine, 5710 Norman Smith Hall, Orono, ME 04469), and P. D. Vaux (Senator George J. Mitchell Center for Environmental and Watershed Research, University of Maine, 5710 Norman Smith Hall, Orono, ME 04469). Stressors and threats to the flora of Acadia National Park, Maine: Current knowledge, information gaps, and future directions. J. Torrey Bot. Soc. 139: 323–344. 2012.— Acadia National Park is a center of plant diversity in northeastern North America. The Park's varied habitats and flora are sensitive to a number of natural and anthropogenic perturbations. Stressors such as invasive plants, pest and pathogens, ozone, acidic fog and sulfur deposition, nitrogen deposition, heavy metals, fire and fire suppression, over-browsing, visitor use, hurricanes, and climate change have all had effects on the Park's habitats and plant species at some point and it is unclear how many of these stressors are currently affecting the flora of Acadia National Park. We discuss the botanical diversity of Acadia, assess the natural and anthropogenic stressors and threats affecting the Park's flora, and summarize critical information gaps to better assess the known stressors and threats to the flora. Understanding these stressors and threats is critical to making informed management decisions to preserve the botanical diversity of Acadia and other regional parks.