Mop-up campaigns are implemented in a country when the final pockets of poliovirus transmission have been identified with certification-standard surveillance. The campaigns involve door-to-door immunization in high-risk districts where the virus is known or suspected to still be circulating. Priority districts include those where polio cases have been found over the previous three years and where access to health care is difficult. Other criteria include high population density, high population mobility, poor sanitation, and low routine immunization coverage.