The Northern Appalachian-Acadian Ecoregion encompasses over 330,000 square kilometers (nearly 82 million acres) in the northeastern U.S. and southeastern Canada, including all or a portion of western Massachusetts, northern New York, Vermont, New Hampshire, Maine, southern Québec, New Brunswick, Nova Scotia, and Prince Edward Island. Ecologically diverse, it is dominated by spruce-fir and northern hardwood forests, extensive coastlines, inland mountain ranges, and glacially carved landscapes. It is also an ecological transition zone between northern boreal and southern temperate forests.

The NAPA is the most intact ecoregion in the eastern United States and contains the broadest extent of nearly contiguous natural forest. Forest cover has been increasing since extensive deforestation in the 19th century so that the remaining areas with over 80% natural cover amount to over 50% of the region. It also contains diverse aquatic, riparian, wetland, and coastal habitats, including floodplains, marshes, estuaries, peatlands, and sandy beaches. There are still large, intact forest blocks in the NAPA with few roads and diverse flora and fauna.