Frankfurt Airport is one of the largest airports in the world and the primary gateway to Germany although Frankfurt is not the capital city. I have been through the airport a few times. Just like other German airports I have been through (like Berlin Tegel, Dusseldorf, and Munich), the Frankfurt Airport is very clean and extremely efficient in processing. The airport has two terminals and it is the hub of Lufthansa. Unlike some of the other European airports which have a separate terminal for departure and arrival for the US, the Lufthansa uses the Terminal 1 and it seems like the terminal is divided into the airline alliance. So, the United is at the Terminal 1 (Star Alliance) while Delta and American use the Terminal 2.