This document summarizes a mixed methods comparative case study approach to understanding what drives routine immunization system performance in Africa. The study examined countries in Africa that had improved DPT3 immunization coverage at the district level to identify factors contributing to increased performance. Through interviews, focus groups, and data analysis at the district, facility, and community levels, the study identified four direct drivers of improved routine immunization coverage: a cadre of community-centered health workers, immunization services tailored to community needs, strengthened health system and community partnerships, and regular review of program and health worker performance.