Madrid airport

Here is info from p. 513 of Rick's Spain book: Madrid’s Barajas Airport
Ten miles east of downtown, Madrid’s modern airport has four terminals. Terminals 1, 2, and 3 are connected by long indoor walkways (about an 8-minute walk apart) and serve airlines including Delta, United, US Airways, and Air Canada. The newer Terminal 4 serves airlines including Iberia, Vueling, Ryanair, British, and American, and also has a separate satellite terminal called T4S. To transfer between Terminals 1-3 and Terminal 4, you can take a 10-minute shuttle bus (free, leaves every 10 minutes from departures level), or take the Metro (stops at Terminals 2 and 4). Make sure to allow enough time if you need to travel between terminals (and then for the long walk within Terminal 4 to the gates). For more information about navigating this massive airport, go to www.aena-aeropuertos.es (airport code: MAD).
International flights typically use Terminals 1 and 4. At the Terminal 1 arrivals area, you’ll find a helpful, though privately run, English-speaking Turismo Madrid TI (marked Oficina de Información Turística, Mon-Sat 8:00-20:00, Sun 9:00-14:00, tel. 913-058-656), ATMs, a flight info office (marked simply Information in airport lobby, open daily 24 hours, tel. 902-353-570), a post-office window, a pharmacy, lots of phones (buy a phone card from the nearby machine), a few scattered Internet terminals (small fee), eateries, a RENFE office (where you can get train info and buy long-distance train tickets, daily 8:00-21:00, tel. 902-320-320), and on-the-spot car-rental agencies. The super-modern Terminal 4 offers essentially the same services. Luggage storage (consigna) is in Terminal 2, near the Metro exit. Some buses leave from the airport to far-flung destinations, such as Pamplona (see www.alsa.es; buy ticket online or from the driver).

There is more than one. Furthermore, terminals 1 and 2 are connected to each other, and terminals 4 and 4S are connected to each other. But between these two groups, you have to take a shuttle bus. And while 4 and 4S are considered "the same terminal" by the airport, they are each huge and there can be a LOT of walking between them (or even within them).

Here's the list of airlines and which terminals they use:
http://www.aena-aeropuertos.es/csee/Satellite/Aeropuerto-Madrid-Barajas/en/Page/1049727006273//List-of-airlines.html

Edit: cross posting with Laura who gave more detail, but you'll find the airlines list useful.

The Barajas Airport site has a map with a listing of the airlines and the terminals they use. More than one terminal, maybe all even, are international fight handlers for one airline or another...

Thanks everyone for the good information on Madrid airport. We are now able to go ahead with our planning .