To fill the knowledge gaps about what lessons have been learned about integration of primary health care (PHC) services in low- and middle-income countries and inform current and future efforts to improve the integration of PHC and immunization services, MOMENTUM Country and Global Leadership conducted this literature review to synthesize what is known about the level of implementation of immunization and PHC service integration in LMICs, identify policy and performance gaps in integration efforts, describe impediments and enablers of integration, and the solutions to address the main challenges.