BACKGROUND: Many children are undervaccinated at the time of hospital admission. Our objective was to explore the facilitators and barriers to vaccinating during hospitalization. METHODS: We conducted qualitative interviews of parents, primary care pediatricians, emergency department (ED) physicians, and pediatric hospitalists. Parents of undervaccinated hospitalized children who were admitted through the ED were invited to participate. We used purposive sampling to identify physician participants. Semistructured interviews querying participants' perspectives on hospital-based vaccination were audiorecorded and transcribed. Parent demographics and physician practice characteristics were collected. Transcripts were analyzed for facilitators and barriers to vaccinating during acute hospital visits by using inductive content analysis. A conceptual framework was developed on the basis of the social ecological model. RESULTS: Twenty-one parent interviews and 10 physician interviews were conducted. Of parent participants, 86% were female; 76% were white. Physician participants included 3 primary care pediatricians, 3 ED physicians, and 4 hospitalists. Facilitators and barriers fell under 4 major themes: (1) systems-level factors, (2) physician-level factors, (3) parent-provider interactional factors, and (4) parent- and child-level factors. Parent participants reported a willingness to receive vaccines during hospitalizations, which aligned with physician participants' experiences. Another key facilitator identified by parent and physician participants was the availability of shared immunization data. Identified by parent and physician participants included the availability of shared immunization data. Barriers included being unaware that the child was vaccine-eligible, parental beliefs against vaccination, and ED and inpatient physicians' perceived lack of skills to effectively communicate with vaccine-hesitant parents. CONCLUSIONS: Parents and physicians identified several key facilitators and barriers to vaccinating during hospitalization. Efforts to provide inpatient vaccines need to address existing barriers.