If you ask U.S. passport holders how long their passport is good for, the majority will say 10 years. While that is literally true, in actuality a U.S. passport is only good for 9 1/2 years because of the "six month" rule. Many countries require that your passport be valid for six months beyond the END of your trip. If it isn't, you will not get past the airline check-in counter.
To learn about a country's entry requirements, U.S. citizens can check the State Department website. There is Country Specific Information listed under its International Travel section. Click on the specific country or countries you will be visiting and then click on Entry/Exit Requirements for U.S. Citizens.
In addition to the expiration date, U.S. passport holders should be aware of the number of free pages in their passport. Check your passport to make sure that you have enough empty pages for the number of expected entry/exit stamps and/or visas. Ignore the pages marked Amendments; they can't be used for these types of stamps.
If you are running out of pages and your passport has several years before it expires, you can return your passport to the State Department to have extra pages added to it. Currently there is an $82 fee for this service.