Most people use their own car but this is a big problem because parking is limited and expensive. You can find a spot at the street but you will never be sure if this is a no parking zone or a zone you will need to buy a parking ticket for. In rare but not too rare occasions your car might be towed away. Nevertheless you might still see other cars next to where yours used to be parked illegally or even ( very common ) on the sidewalk.
To add to the problem not all cars can enter Athens every day. There is a parametric zone called "daktilios" or "ring". In order to enter this zone during the weekdays till 8 pm your should check your license plate last digit. IF the day of the month is an even number then only cars with last digits of even numbers can enter and vise versa. As long as you rent a car and you have a license from another country and you are not a permanent Greek resident you can enter the zone any day. If you are caught inside the zone on a wrong day you have to pay a ticket.
This stupid measure lead people into buying a second car with the number it fitted them.
There are many places in Athens that you can find a rental car agency. Most, including the well known agencies, have offices at the start of the "Singgrou " avenue. The most common rental is a small car because it can fit everywhere. Now days they offer automatic cars with air-conditioning and electric windows.

Athens has a bus service called ETHEL. Actually there are many buses that they can take you anywhere you please. The only problem with this service is that the buses don't run a a tight schedule so the service is unreliable. Buses don't stop when they see you at the bus stop but you need to do a hand signal like you are trying to stop the bus with your arm. This is because I believe the same bus stop services many routes so the driver doesn't know if you are there waiting for his bus or another.
Athenian bus drivers have a bad reputation for been rude, they talk at the cell phone while driving and sometimes they don't stop to pick up older people. This is because older people delay the bus while trying to ride it making them work longer hours. Handicap people is almost impossible to ride a bus.
You need to have a pre-bought ticket of 0.5 Euros in order to ride a bus and finding a place to buy one, let say in the evening, is not easy. You validate your ticket inside the bus. If the machine is not working make sure you have told the driver because without a validated ticket you might pay a fine.
You can buy though a monthly or day pass that is good for all buses and even all transportation means except taxis.
Recently the whole bus fleet was upgraded with newer and less polluting vehicles. There are two kind of buses, the blue buses that run or petroleum and the Green buses that run on natural gas. Some buses have air-conditioning running the hot days of summer.

Similar to the buses are the "trolleys" or ILPAP. The Athens trolley is basically a bus that gets it's power from an electrical grid above the street. Theses buses are yellow and they are as unreliable as the other buses. Some have air conditioning which is a must during the summer days and are able in case of a power failure to continue with a gas engine. Trolleys are slower than the buses because they have to stick to the power grid thus they can't easily pass other slower cars, other buses or trolleys. This doesn't mean that they might not try it. The trollay ticket is also 0.5 Euros,

Athens now days has a well maintained and rather reliable underground grid of trains called the "Atikko Metro" There are two lines the red line and the blue line and they
are expanding and now are servicing many areas. The metro stations are generally clean and nice but not all trains have air-conditioning and the stations are rather hot the summer days. At most station you can see information on how long you have to wait for the next train.
You can buy a ticket for 0.80 Euros at the station from either a teller or a machine and you can validate it before you enter the station. There is a fine if you are caught without a validated ticket and in many cases I have see officials checking the passengers in various points, so never throw your ticket away unless you have completely. exited from the station completely.
There is also an additional line called "Electrikos" or ISAP that was operating since the fourties and now is also called the green line. These trains are older but some have air-conditioning. All their stations have recently been renovated but still they look old and are not as clean and nice as the " Atikko Metro" stations. The same ticket regulation apply here.
A ticket form the "Atikko Metro" is also good for "Electrikos" but not vice versa. Normally a ticket costs from 0.7 or 0.8 Euros.It is a good Idea if you are buying the ticket form a teller to say where you are going so you get the right ticket since the route is divided into zones and not all zones cost the same.

The "tram" This is the latest and rather unsuccessful addition to the system. The tram is a surface street train that moves in slow speeds and in this case does too many stops. Right now there are 47 stops so it is sometimes slower than a bus. The tram is worth a trip because it goes through neighborhoods that travelers don't normally visit but this means you will go through some rather tight streets. A great amount of the ride is coastal though.
A ticket for the 26 kilometer route costs 60 cents. If you are transferring from another form of public transportation like the metro, buses or trolleys the cost is 40 cents. People under 18 travel for 40 cents and the disabled travel for free.

During the Olympic games an additional line was introduced called the 'Proastiakos". This is a suburban railway that connects the airport with the city and the city of Corinth. This is also expanding. There are different ticket according where you want to go and the stations are a bit confusing since the trains move back and forth on the same rails.

Yes the Athenian TAXI is worth of his own lens! Find more complete information for this beast at: How to hire a TAXI in Athens Greece.