Immunization is a program whose success or failure in achieving public health impact is often judged on the basis of data generated on a daily, weekly, or monthly basis by health workers in facilities ranging from remote health posts to regional hospitals. Because immunization is a service that can be scheduled (unlike, for example, treatment of ill children) its data can be used by health workers to answer such key management questions as: Are we reaching the people in each locale who need immunization? Are we making progress? Who are we not reaching? Are children starting the immunization schedule but then dropping out? How well do outreach services work? In principle but not always in practice, the answers to these questions can be found by health workers themselves with data that are readily available to them. The importance of data use is highlighted by the inclusion of facility-level microplanning as a core component of the WHO/UNICEF Reaching Every District strategy, used by most countries to strengthen routine immunization (RI).