Despite marked progress in global polio eradication, the threat of polio importation into the United States remains; therefore, all children should be protected against the disease. The standard schedule for poliovirus immunization remains 4 doses of inactivated poliovirus vaccine at 2, 4, and 6 through 18 months and 4 through 6 years of age. The minimum interval between doses 1 and 2 and between doses 2 and 3 is 4 weeks, and the minimum interval between doses 3 and 4 is 6 months. The minimum age for dose 1 is 6 weeks. Minimal age and intervals should be used when there is imminent threat of exposure, such as travel to an area in which polio is endemic or epidemic. The final dose in the inactivated poliovirus vaccine series should be administered at 4 through 6 years of age, regardless of the previous number of doses administered before the fourth birthday, and at least 6 months since the last dose was received.