If you use it as a verb, there is no hyphen. I have to check in at the desk. [To see this more clearly, use past tense - "I checked in at the desk." You would not say "I checked-in at the desk."] If you use it as a noun or an adjective, use the hyphen. I have to go through check-in. I'll be at the check-in counter.

Elizabeth, Verb I check in at the airport...... correct. Check in is a phrasal verb and does not need a hyphen. I check-in at the airport.....incorrect. Noun With a hyphen "check-in" is a noun that means "the act of checking in" or the "place where you check in." I arrived at the check-in when the plane was taking off. Check-in is at 13:00. Check-out is at 11:00. Adjective Check-in can also be an adjective. Check-in time is 13:00 Oops, fdmaxey already answered this one. :)

I check in at the airport. verb = check in noun = check-in That's the difference.

You can use both. But it´s more commom use check-in.