On February 14, 2024, MOMENTUM Routine Immunization Transformation and Equity held a learning exchange webinar to discuss how countries operationalize and measure the definitions of zero-dose and under-immunized children. This learning exchange discussed the operational definition of zero-dose children, followed by experience sharing from Mozambique, Bangladesh, and DRC. Country presenters discussed the issues they faced with operationalizing the standard definition and how they are moving forward. The examples featured a range of use cases and discussion with joint problem-solving.