In a 2023 sequential explanatory mixed-methods study in four US states, we identified barriers and facilitators experienced by Vaccines for Children (VFC) program providers in reporting vaccination data to state immunization information systems (IISs). We found the following: VFC providers value accurate, robust, and widely used IISs. IIS reporting is easier with but does not require an electronic health record. Negative interactions with IISs and VFC officials and limited practice capacity are barriers to reporting. The COVID-19 pandemic highlighted the need for a nationwide vaccination database. (Am J Public Health. 2024;114(6):569-574. https://doi.org/10.2105/AJPH.2024.307638).