If you were near a person who later tests positive for an infectious disease, an exposure notification app can let you know. These apps allow for more rapid and broader contact tracing—ideally helping to slow disease spread. About half of the U.S. states had their own COVID-19 exposure notification apps as of June 2021. A national app wasn't available. We found 5 challenges related to exposure notification apps, such as privacy concerns and a dearth of evidence showing that the apps are effective. To address challenges, we identified 4 policy options, e.g., collaborating on a national strategy for these apps.