Using a first-difference econometric model, we estimate an aggregate demand model for assessing the determinants of the quantity of visits to the 47 national parks in the continental United States. The estimated model was then used to project visitation to these parks from the 2016 base year to 2026. Total visitation could see an average increase of about 1.2 million visitors per year through 2026, suggesting that congestion problems already experienced at many parks may get worse. Congestion and overuse strain already limited operation and maintenance budgets and can lead to environmental damage to park sites and reductions in visitor satisfaction.