Using root cause analysis and a rapid evidence synthesis, this brief describes how to overcome a key obstacle to reaching zero-dose children, which is the lack of operational funds at the sub-national and facility levels to implement tailored delivery and advocacy strategies. It identifies the root causes of this challenges while also synthesizing the evidence on the marginal costs of reaching zero-dose and under-immunized children and the best practices for planning and budgeting for immunization activities to ensure that the operational costs required to reach zero-dose children are accounted for in sub-national budgets.