There are several options for getting from the Athens airport to downtown but some are better than others.  Most hotels do not provide transfers from the airport to Athens, but it’s worth asking.  Renting a car is not necessary if you’re only planning to do sightseeing within the city. Athens sights are easily accessible by foot, taxi and Metro. For a foreigner, traffic and street signs are difficult in downtown Athens.  Parking could be a problem, so research the neighborhoods your visiting, to determine your best options.

That said, there are basically three options to transfer: taxis, buses and the Metro.  Taxis queue outside the terminal. It’s worthwhile to have your hotel name and address written in Greek. While taxis run the meter (ask them to reset it when getting in), the charge can still vary significantly.  Basic rate to city centre by taxi is 35 E daytime, 50 E night time.

The Metro is quite easy to use and the charge is consistent.  The train station is connected to the airport, but does require going up a level from arrivals (by elevator) and crossing over a pedestrian bridge. (The taxi stand is closer to the arrivals area.)  Tickets can be purchased from a kiosk or a person at the desk in the station.  One way is 10 Euros per person. It may make sense to purchase a 3 day tourist pass which includes round-trip airport transit plus local Metro. http://www.athenstransport.com/englis...

It is very clean and new, a result of the Olympics.  Stops are well marked and announced ahead of time. The time between the Monastiraki station (one of the primary downtown stations, near the Parthenon) and the airport is approximately 45 minutes.  There is an escalator to the street level.

In front of the Airport there is a public transport express bus station, convenient especially for those traveling further from Athens. There are 5 different lines and for those who are trying to catch a bus (KTEL, regular regional bus lines throughout Greece) X93 line is driving directly to Kifissos main bus station for Eur =3,60 per person (gone up to 5 euros in March 2011) and it takes approximately 45 min to 1 hour to get there. **** X95 to Syntagma Square - City Centre, and x96 via Glyfada to Piraeus. Cost 5 E,  seniors 2.50 E. Busses could be old and not very comfortable.

For additional information about all the options, see the following English site:

athens airport

Athens airport transportation maps