Athens is served by the Athens International Airport "Eleftherios Venizelos" (airport code ATH) which is the largest airport of Greece and the only international airport of Athens.
The airport is accessible by metro, buses, taxis, and the suburban railway.
Metro is perhaps the easiest option for traveling from the airport to the city center (Syntagma Square) through Line 3 (or blue line). The trip from the airport to Syntagma station is approximately 40 minutes, with departures every 35 minutes (Line 3 timetable information). The fare costs 9EUR (16EUR for a round-trip ticket).
Bus X95 is also a very convenient way to reach the city center. The trip duration from the airport to Syntagma station is roughly 40 minutes (note that the actual duration depends on the traffic), with buses departing every 15-20 minutes (X95 timetable information). The fare costs 5,5EUR.
Taxis from Athens International Airport to the city center (Syntagma Square) have a flat rate of 40EUR during the day and 55EUR at night. You should expect to pay ~10EUR more to directly reach the conference venue from the airport.
You can also use the Free Now, Taxiplon, and Uber online applications for booking taxis.
The conference venue is 10 minutes walk away from the Sygrou-Fix metro station. If you want to go from Syntagma Square to the conference venue by metro, you can get metro Line 2 (or red line) towards Elliniko, get off at the Sygrou-Fix metro station, and walk.
Alternatively, the bus stop Gephyra is 1 minute walk away from the conference venue and is reachable by many city buses (e.g., 040, A2), while the tram stop Kassomouli is 2 minutes walk away and is reachable through Line 6.
Finally, a taxi is the fastest option and should take ~10 minutes, but the duration depends on traffic.
The mass transportation system of Athens consists of metro, city buses, tram, and the suburban railway. Detailed information regarding the ticket options and prices can be found here.