!!! A strike for Metro and Tram has been announced for Wednesday 1/3 and Friday 3/3. In case you are arriving or leaving on these dates it is advised to use the bus for your transfer !!!
Location
Conference will take place at Electra Palace hotel. GPS DD (Decimal Degrees) coordinates: Latitude: 37.973778, Longitude: 23.731654
Access from "El. Venizelos" Athens International Airport
By Metro (suggested, running time 40min), First train is at 6:30am, Last train is at 11:30pm. See here for timetable
Take metro line 3 (blue line) from "El. Venizelos" Athens International Airport Station to Syntagma Station in the direction of Asia Marina. 
The access to the train station and ticket counter is at the departures level (i.e. "First Floor" of main airport building). At arrivals, you can take Exit 3, cross the road and use the electric stairs to climb one floor or use the elevator if needed. At the station buy your ticket, validate it and then take the stairs to the departures platform. There are two lines departing from the station, the suburban line and the metro line (a blue m in a green circle). Ask for the metro line. If you take the suburban train there is no problem! Get off at Plakentias station and continue with blue line from this station. (Until this station they run in parallel).
Ticket cost for single ride 10€, it is cheaper though to buy return ticket or tickets for group of three etc. Please see also here for all the available ticket types. There are 3-days tickets that include also the ride to and from the airport or 5-days tickets (that do not include the ride to and from the airport).
By Bus (running time 70min), Busses from airport run 24hours. See here for timetables
Take bus X95 to Syntagma.
If your destination is not Syntagma square and X95 line does not suit you you can take line X93 line (direction Kifissos) get off at Kifisias ring. You should walk around 100 meters the same direction to catch the next bus stop on the ring and from there you can take busses A7, X14, 550.
Busses are at the arrivals level. Take Exit 5 of the departures level. Buy you ticket before boarding the bus. but please see also see here for all the available ticket type. There are 3-days tickets that include also the ride to and from the airport or 5-days tickets. Single ride ticket cost 6€.. See also here for further details
By Taxi (running time around 45min): Taxis are available at the designated Taxi waiting area is located between Exit 3 and Exit 2 of Arrivals Level. From/To the airport and to/from city centre there is a flat fare of 38€ (5am to midnight) / 54€ (midnight to 5am). The fare includes everything (any number of luggages, tolls, etc). If you choose to use Taxi do not pay anything else whatever they tell you and ask for a receipt. It you wish you can leave a tip.
From Syntagma metro station or from terminal of X95 bus hotel is very close. Please see map below for your reference
Tips and tricks
- Use Google maps or Here maps for your transportation. Athens public transportation has been integrated into them
- There are two available apps in Google Play / iTunes for public transportation. "TFA Tickets" is for using electronic and not paper tickets. "OASA telematics" has real time information of public transport in the vicinity
- Especially metro and sometimes buses can be very crowded. Beware of pick-pockets!
- Whenever you use a taxi there must be a meter running (except for the flat rate between the airport and city centre). For an indicative list of costs please see here. Taxi drivers need to issue a receipt each time and they are not allowed to take double routes (for the same route get paid double from two different passengers).
- Taxis do not accept credit cards (except if you book them online). You pay by cash. Be aware always of how much you paid and if they tell you differently do not accept it