Customs

Once you're in a terminal in Atlanta, you remain within the secure zone the whole time. In thirty minutes you can stroll between the most distant gates, take a restroom stop, and have a cup of coffee. Twenty is comfortable. Fifteen sucks, but we do it every once in a while. The above times start once you clear the plane.

You will go through passport control and customs at the Paris airport. In Atlanta, if your connection involves only one airline, an hour should be adequate. If you are changing airlines, and possibly terminals, you may need more than an hour. In fact, even the same airline may have domestic and international terminals, so some transit time may be required getting one to the other.

I would allow at least 1 1/2 - 2 hours between flights in Atlanta. It's a large airport and you will have to take the train between the domestic terminal and the international terminal. This could take you 30 minutes or more, depending on which terminal you land in. Taking into consideration that international flights start boarding 45 prior to departure, you will have killed well over an hour before departure just getting from one plane to the next and boarding. You do not go through customs in ATL but you do have to get your passport verified by the gate agent prior to boarding your flight. There are plenty of restaurants / bars in the airport to kill some time if you are too early. you don't want to be late!

Further info about ATL can be found here: http://www.atlanta-airport.com/ The airport has 7 concourses and Delta uses most of them. For example, if you arrive at Concourse A, you will have to deplane, go down the escalator to the train, take the train to Concourse F and then the escalator back up into Concourse F to find your gate. Trains run frequently so you won't wait more than a few minutes. Each concourse is huge...they have anywhere from 34-48 gates each.

I always allow a minimum of two hours and have never had my luggage fail to arrive at my final destination.