The United States has been issuing passports with electronic chips in them since late 2006. Visitors to the United States have been required to have chips in their passports since 2002, however.
According to Homeland Security, the electronic chip inside a passport contains all of the information contained within the passport, including a digital image of the passport holder. This is a security measure that makes passports more difficult to alter as well as for stolen passports to be successfully presented to verify identity of anyone to whom they do not belong. The data on the chip can be compared to the passport as well as the individual presenting for admission into or out of the United States.