Introduced forest pests and pathogens are a major threat to national parks. This paper reviews existing impacts, projected impacts, and management options for Tsuga canadensis (Eastern Hemlock) forests in US national parks threatened by the introduced insect Adelges tsugae (Hemlock Woolly Adelgid [HWA]). Eighty-five national parks, or 21% of all parks in the US national park system, are encompassed within the range of Eastern Hemlock. These 85 parks include iconic areas such as national battlefields and the Appalachian National Scenic Trail. Four focal parks of this study—Delaware Water Gap National Recreation Area, Great Smoky Mountains National Park, Shenandoah National Park, and New River Gorge National River—are collectively visited by 16 million people annually and Eastern Hemlock is a forest component in 2–26% of their area. Results of research on HWA impacts to forest species composition, soil nutrient cycling, watersheds and fisheries, wildlife habitat, and visitor experiences and safety have already been reported from these parks. A general principle is that after Eastern Hemlock forest decline, some species (e.g., some avian species favoring other tree species) benefit, while those associated with Eastern Hemlock must adapt or decline. Forecasting future forest-tree composition is complicated by the fact that: (i) many possible replacement tree species are themselves threatened by introduced damaging agents, (ii) changes hinge upon understory dynamics such as invading exotic plants or expansion of native shrubs, and (iii) this die-off event is occurring within a context of multiple interacting factors such as elevated herbivory, climate change, and atmospheric pollution. Some management strategies for parks include: intensive HWA chemical treatment at priority sites, biocontrol, genetic manipulation for HWA resistance in Eastern Hemlock, exotic plant treatment, facilitated establishment of native vegetation, or doing nothing, the last of which also is likely to result in appreciable forest change. Threats to US national parks posed by introduced forest pests and pathogens warrant heightened attention.