By bus
Cologne has an international bus station ZOB; it is located at Breslauer Platz on the north side of the Hauptbahnhof (http://www.deutsche-touri...). Direct daily buses to Serbia and alternate daily buses to Montenegro.
By train
Cologne is served by two major train stations - Köln Hauptbahnhof (http://www.bahnhof.de/sit...) and Köln-Deutz.
Cologne is linked with Amsterdam, Brussels, and Paris by Thalys and ICE High Speed trains. Additionally, the Frankfurt airport IATA:FRA has direct service to Cologne and is within one hour by ICE trains.
By car
As of January 1, 2008, Cologne requires all cars to have a "Low Emissions" sticker in order to drive around in the city center Low Emission Zone, "Umweltzone". Information on obtaining a sticker which must be done at least several weeks in advance is available here (http://www.stadt-koeln.de...).
Plenty of motorways Autobahns A1, A4, A3, A57, A555 lead to Cologne. During rush hour the streets are heavily congested, also due to massive construction of a new subway tunnel Nord-Süd Stadtbahn, crossing half the city centre.
For cheap parking, with quick connections to central cologne, use park and ride "park und ride". At some stations, parking is free when you present a validated transit ticket on exit. More info can be found here (http://nicecity.de/englis...).
Find a street map, also showing where you can switch into a KVB subway, S-Bahn or Bus here (http://www.hot-maps.de/eu...).