This year (2016) is the 10 year anniversary of the initiative which requires American citizens to have a passports for traveling to Mexico and the Caribbean.
Since there were a large number of passports issued 10 years ago, these passports will need to be renewed this year. This could mean delays for you if you are renewing your passport or planning to get a new passport. Even under normal circumstances it can take a couple of months to get your passport! So be sure to plan ahead!
If you already have a passport make sure it is valid for the dates you expect to travel. Some countries will require your passport to be valid for six months after travel and you may also be required to have a visa for certain countries, so be sure to check with the US State Department before you book your flight: https://travel.state.gov/content/visas/en/visit.html
If you’re looking to renew your passport or getting a new one, you can find passport applications at your local US Post Office. You can also take your official passport photos there, too!
Need a passport in a hurry? Expedited services ARE available and you may be able to get it as quickly as three weeks. For more information, go to the official US Department of State website at: https://travel.state.gov/content/passports/en/passports.html
EDIT 04/07/2017- 2016 may be over, but if you’re looking for a US Passport, please go check out the Travel.State.Gov for information.