Sustainable financing for immunization refers to the sufficient and predictable allocation and use of resources to support the achievement of immunization goals within the framework of overall health financing. The Immunization Agenda 2030 (IA2030) agenda spells out four important focus areas needed for sustainable financing: (1) ensuring sufficient and predictable resources, (2) making optimal use of resources, (3) aligning partnerships, and (4) supporting sustainable transitions from external assistance. This paper summarizes the evidence and proposes interventions under each area. While immunization is one of the best investments and justifies public financing, the COVID-19 pandemic has led to the worst economic recession since the Great Depression and threatens countries' ability to mobilize funding to ensure continuity and access to essential services, including immunization. Strategies for ensuring adequate resources differ by income group but include raising more revenues, reprioritizing the budget towards health, and ensuring that health resources favor Primary Health Care (PHC) and immunization. In low- and lower-middle income countries, support from Gavi, the Vaccine Alliance, which channels the largest amount of external financing, will remain important, but some lower-middle income countries will need to prepare for transition. Countries benefitting from the Global Polio Eradication Initiative (GPEI) are also experiencing a transition from GPEI financing to domestic and other external financing. This paper outlines ways in which countries can improve the use of domestic and external resources to better incentivize high-quality PHC and immunization services and align immunization programs with health sector reforms. While governments must lead, collective action from development partners, the private sector, and civil society is needed to promote health system financing systems that ensure that the world is better prepared for future outbreaks and pandemics, while reinforcing the IA2030 vision and making progress towards universal health coverage and the Sustainable Development Goals.