Only iPhone 5 smartphones sold between September 2012 and January 2013 are eligible, and only those that fall within a certain range of serial numbers. Apple has opened a website that allows people to determine whether their phones are eligible. (To access your serial number, tap Settings > General > About > Serial Number).
Apple announced an even smaller iPhone 5S battery recall program last year. In June, Apple issued a recall in 37 countries for European iPhone chargers that were overheating.