Immunization represents one of the most effective tools in preventive medicine. But despite what should be a universal practice, preschool children, particularly in the inner cities, are not being adequately vaccinated. The responsibility for low immunization levels does not rest solely with the parents. Major obstacles within the health care system provide disincentives to immunization. Local resource problems such as inadequate clinic staff, hours, and locations make immunizations difficult to obtain. When comprehensive care is not easily accessible (e.g., requiring appointments weeks or months in advance), policies which require immunization to take place only within such a setting are further barriers. Many opportunities for vaccination are lost when children interact with the health care system but do not receive all the immunizations they need. Policies must be changed to facilitate immunization and to take advantage of all health care visits to provide vaccines.