This map from the U.S. Geological Service shows the range of the volcanic ash and debris that was deposited after the three large Yellowstone volcano eruptions over the last 2.1 million years. as well as the Mount St. Helens eruption. (USGS)
The possibility of the volcano under Yellowstone National Park erupting has increasingly caught the attention of people around the United States (and even other parts of the world), and a number of discussions have centered on potential action plans if an eruption does happen.
Note that these discussions are purely hypothetical and that most scientists have emphasized that the chances of Yellowstone having a major eruption again anytime soon are actually diminishing.
At the same time, several agencies and universities do constantly monitor signs from the caldera to be able to warn people if there is an eruption. And if an eruption did happen, it could cover half of North America in ash and debris, with winds possibly distributing it even further.
First, some survival blogs suggest stocking up on supplies if you live in an area that would be affected by a large eruption. Supplies include dust masks, goggles, non-perishable food, and drinking water.
Radios, flashlights, and extra batteries, as well as extra blankets and maps of potential places to evacuate to are also considered essential, as are huge tarps that could cover your home and vehicles. Duct tape would be good for sealing gaps between windows and walls, etc.