Always make sure to check the validity of your ESTA before traveling to the U.S. Attempting to enter the United Stated with invalid ESTA will result in entry denial.
ESTA is valid for 2 years or until the expiration of your passport, whichever comes first. During the validity period of your ESTA you may enter the United States for multiple times given that each trip is shorter than 90 days.
Should you change your passport or any other data that cannot be modified ( here you can read about the data that can be edited), your ESTA will no longer be valid. If this is your case, please make a new ESTA application. If you have any doubts, there are procedures to check if your ESTA is still valid.
Note: New rules have been implemented in 2016 regarding ESTA validity. If you are national of a VWP country and are also citizen of either Iran, Iraq, Sudan or Syria you will no longer be eligible for the Visa Waiver Program. Furthermore, if your are from a VWP country and have been present in or traveled to Iran, Iraq, Syria or Sudan on the 1st of March 2011 or after, you will no longer be eligible for the visa waiver. However, exceptions may apply to these changes. Learn more about the new rules for the Visa Waiver Program.
If your ESTA is no longer valid, fill in a new ESTA application form and submit it easily here.