You must have a certain amount of physical presence and continuous residence in the United States to be eligible to apply for U.S. citizenship.

(1) Must have certain physical presence in the U.S.

You must be physically present in the United States for at least half of the required years of continuous residence. That's at least two and a half years before you file your petition and before you take your oath, if you need to complete five years as an LPR (Legal Permanent Resident). It's one and a half years for spouses of U.S. citizens who need to complete three years as an LPR.

The day you leave the U.S. and the day you return are counted as days of physical presence in the U.S. for citizenship purposes. The days do not have to be consecutive in any way. USCIS will simply look at the last five (or three) years and count the number of days you spent in the United States. Even if you have maintained continuous residence by not being outside the U.S. for more than six months or a year at a time, several shorter trips could be a problem for the physical presence requirement.

(2) You must have a certain amount of continuous residence in the U.S.

The general rule is that just before you apply for U.S. citizenship and before you take the oath of citizenship, you must have five years of continuous (uninterrupted) residence in the United States. The rule changes to three years for people who are married to and living with a U.S. citizen spouse at the time they're sworn in as U.S. citizens.

To be considered continuously present, you must have lived in the United States on a permanent basis. USCIS looks at where you have actually lived. If you've lived overseas, it doesn't matter if you considered the United States your primary residence or if you always planned to move back.

If you have been out of the U.S. for more than six months in the five years prior to filing your petition, USCIS will consider you to have broken your continuous residence! During the citizenship application process, you may be able to convince USCIS that your absence didn't break your continuous residence. To do this, you'll need to show that you kept your job in the U.S. and didn't take a job overseas while you were away, that your family stayed in the U.S. while you were away, that you kept a place to live in the U.S. when you returned, and other evidence of your ties to the U.S.

If you were absent for a continuous period of one year or more during the five (or three) year period, your continuous presence was broken. You can't try to convince USCIS otherwise. Before you can apply for citizenship, you must wait until you've been back in the U.S. for four (or two) years and one day. You don't have to wait the full five (or three) years. But even if you do leave, it's best not to go away for more than six months.

(3) Typically, the biggest hurdle is when you actually return to the United States and interact with a Customs and Border Protection (CBP) officer. 

The officer who greets you will ask questions to determine if you have abandoned your permanent residence if you took a trip for more than six months, and you really have some explaining to do if your trip lasted more than a year. The CBP officer will look at the reason for your trip, how long you intended to be gone, and what caused you to be gone so long. You can also abandon your permanent residence by taking multiple trips, no matter how long, if you don't really spend much time in the United States overall.  

But the fact that you made it back to the United States is no guarantee that USCIS won't take another look at your naturalization application and decide that you did, in fact, abandon your U.S. residence. For more information, see how Don't Lose Your Green Card Due to Long Absence from the U.S.

Tips:

You must decide where your U.S. citizenship interview and oath ceremony will take place, and live in that state or USCIS service district for at least three months before filing your U.S. citizenship application. If you move later, you can change your interview location.

You must be in the U.S. for your fingerprinting appointment, citizenship interview, and oath.

What if you're stranded outside the U.S. and the total days of your absence add up to the required period of physical presence? One option is to quickly file your naturalization application from abroad so that your time is counted from the date your application is received by USCIS.