Getting to Heidelberg
Heidelberg's city homepage has (vague) information (available only in German) here (Google Translate). There is also information from the original DC15 bid, as well as Arrival information at the venue website.
The following is an attempt to collect all relevant information in one place.
 Travelling by airplane
The nearest aiports for Heidelberg are Frankfurt (FRA) and Karlsruhe/Baden-Baden (FKB), also known as Baden-Airpark. The first one is an international hub having flights from/to anywhere in Europe and the world and the latter one is a small regional airport serving mainly low-cost airlines (Germania from/to Tel Aviv, Ryanair).
Other options include:
- Frankfurt Hahn Airport (HHN) is served by Ryanair, although it lies very remote and has nothing to do with the "real" Frankfurt airport.
- Basel/Mulhouse/Freiburg (BSL) in the three country region France/Germany/Switzerland + train (see #train Train). Basel/Mulhouse is e.g. served by easyJet, connecting with many destinations in Europe.
- Stuttgart (STR)
 From Frankfurt/Main (FRA)
If you are landing in Frankfurt/Main, your best options to continue are either by car (driving instructions) or train.
Trains depart straight from the airport ("Frankfurt Flughafen Fernbahnhof") and run either direct to to Heidelberg Hbf, or via Mannheim Hbf. See #train for more information. The station is 200 metres from the Terminal 1 entrace. A free shuttle bus every 10 minutes can take you there from in front of Terminal 2.
ICE train from "Frankfurt(M)Flughafen" to Mannheim, and S-bahn to "Heidelberg Hbf", takes about 1 hour total and cost around 25 €. If you want to know more about rail fares and options, please see this write-up
 From Baden Airpark (FKB)
As said above, FKB is a small regional aiport not located closely to a bigger city but in small Rheinmünster. Karlsruhe is 40km away while the small city of Baden-Baden is 12km. This needs to be thought of when looking of train connections. You will need to change in either Karlsruhe or Baden-Baden. Use "Flughafen Karlsruhe/Baden-Baden Terminal, Rheinmünster" in the departure field when planning. See #train.
If you are renting a car (especially worthwhile and necessary during early morning/late evening arrival/departure…), here are driving instructions.
You can also take a bus, see here):
- Bus 140 (Hahn-Express) goes from Baden-Airpark over Karlsruhe Main Station directly to Heidelberg Main Station. This is also the option you need to take if you arrive at Frankfurt Hahn because it's terminal stop (as its name already says) _is_ Frankfurt Hahn. From Hahn the bus goes to Heidelberg Main Station via Ludwigshafen and Mannhein. The timetable is on the Homepage. Fare is 20 € one-way, and 36 € round-trip for both FKB and HHN)
- From FKB you can take the public buses 205 and 234 to either Baden-Baden or Rastatt and continue from there via train. Note that Rastatt is only a regional train stop, no high-speed IC/ICE connection. Please see #train for connections from Baden-Baden, Rastatt or Karlsruhe to Heidelberg
 From Frankfurt-Hahn (HHN)
The only sensible option is the bus number 140, the same as for FKB (see above). Travel time is around 2:10 hours.
 From Basel-Mulhouse (BSL)
You need to go to the city of Basel (to Basel SBB or Basel Bad Bf) and take the ICE/EC. See #train.
Bus 50 goes from the Airport to the Basel SBB railway station and vice-versa. See here
 From Stuttgart (STR)
From Stuttgart airport, take S-Bahn line 2 (toward Schorndorf) or 3 (toward Backnang), to Stuttgart Hbf. You can then take an IC train (toward Hamburg-Altona) directly to Heidelberg Hbf, or an ICE train to Mannheim where you can transfer to another train to Heidelberg.
German high-speed trains are called ICE and are similar to the French TGV, however, no reservation is required on ICE trains.
Heidelberg is well connected to the German train network although it has very few ICE stops. But Mannheim and Karlsruhe are near — and those have plenty of ICE and TGV connections. The local trains/S-Bahn take around 20 minutes from Mannheim and 45 minutes from Kalrsruhe to Heidelberg main station.
For those coming out of Switzerland the best option would be to take the ICE. See here (German). There's an ICE connection from Interlaken-Ost → Bern → Basel → Karlsruhe → Mannheim (and further via Frankfurt/Main and Göttingen to Hannover/Hamburg/Kiel or Berlin) and a EC Zürich → Basel → ... which are the best to take. Then you can change in Karlsruhe or Mannheim to a regional train/S-Bahn.
From France the TGV stops in Mannheim and Karlsruhe so you easily can take that one to reach them and getting from there to Heidelberg. See picture here (German, Google Translate) (note the full connection Marseille → Frankfurt is only once per day) but there's also a direct Paris → Strasbourg → Karlsruhe TGV connection in 3hrs. See here for informations in French.
For other neighbouring countries taking the train might also be an option (e.g. Thalys/ICE Bruxelles → … → Köln (Cologne) and then down to Mannheim or Eurostar to Bruxelles or Paris and then onwards via Thalys or TGV/ICE).
 Getting from Heidelberg Main Station to the venue
From the main train station you can reach the youth hostel by bus within 10 minutes. The bus station is opposite the main train station. Take bus line no 32, direction to "Neuenheim (Kopfklinik)". The youth hostel has its own stop Neuenheim, Jugendherberge.
A taxi from the station to the hostel will cost around 12 €.
 By Car
You can reach Heidelberg via motorway A 5 and A 656. From the South A 5 (direction Karlsruhe-Darmstadt) take exit Schwetzingen/Heidelberg. From A5 (Darmstadt-Heidelberg) take exit Dossenheim. Direction Klinikum Neuenheimer Feld. The youth hostel is located directly next to the zoo.
Questions about parking are answered here.
Attention : even though the conference venue is outside the so-called "environment protection zones" and access roads are also exempt, you will need a German environmental badge for your car if you plan to drive into the city. These need to be ordered in advance. More information is available from the FAQs
 By Bike
The hostel is on the Neckertal Radweg, a major cycle route alongside the river Nekar. It is 1.5miles/13 minutes from the Main Train Station. http://www.cyclestreets.net/journey/43789187/
General information about bringing a bike in the train: http://www.atob.org.uk/bike-rail/bikeeurope-germany/