Immunization activities, in spite of being one of the easiest health components to deliver, are frequently forgotten or relegated to a low priority. One of the major reasons why immunization services have not been more widely implemented in developing countries is that present knowledge is inadequately applied. Although gaps do exist in some technical and operational areas, the most important concern is the application on a larger scale of already available knowledge and technologies. It is emphasized that programming strategies should promote delivery of immunization services to the population groups at highest risk of contracting the target diseases -- children less than 1 year old and pregnant women -- and that these services can most effectively be delivered as an integral part of the primary health care system. The use of immunization coverage as a simple and meaningful indicator of the extension of coverage of health services is analysed by using data from immunization programmes in two countries of the Americas.