https://www.aa.com/aa/reservation/flightCheckInViewReservationsAccess.do
It is right on the main page of
www.aa.com. Information pasted from the faq's on the AA site.
What is Flight Check-In?
A: Flight Check-In is a service that enables American Airlines, American Eagle and American Connection® customers to check in for a flight from home, the office or anywhere there is an Internet connection. Additionally, if your itinerary includes a connection to a carrier with which we have an e-ticketing and online check-in agreement (check here for current airlines) you may check in for your flights with these carriers as well.
Simply visit
www.aa.com. Then, in the upper right corner of the home page within the Reservations box, select the Flight Check-In tab and follow the simple instructions.
Flight Check-In enables you to avoid check-in lines at the airport by printing your boarding pass from home and heading directly to security and the gate when you arrive at the airport. Flight Check-In is available for travel within the U.S., including Alaska and Hawaii, Puerto Rico and the U.S. Virgin Islands.
Please note that boarding passes for carriers other than American Airlines, American Eagle and AmericanConnection may not be issued depending on our agreement with the carrier involved. You may still use Flight Check-In for these carriers, but you will need to obtain a boarding pass directly from that carrier.
Q: Who can use Flight Check-In?
A: Flight Check-In is available to customers who are traveling on an American Airlines, American Eagle or AmericanConnection® electronic ticket. You may also use Flight Check-In, if your itinerary includes a connection to a carrier with which we have an e-ticketing and online check-in agreement (check here for current airlines). This electronic ticket is not limited to AA.com purchases; it also includes purchases made through an online agency or any
travel agency.
You cannot use Flight Check-In if you have been issued a paper ticket.
Q: When can I check in for my return flight?
A: AA.com allows you to check in for your return flight, once your outbound flight arrives and you are within 24 hours of your flight.
Q: Will I be able to check in for both my outbound and returning flights, if I return in less than 24 hours?
A: No.