Prerequisite for the use of Citybike Wien was a one-time registration. You could do it at a station or right here online.
After registering, you could rent a bike at any Citybike station in Vienna. Around the clock, the whole year.
Returning a bike was easy: just insert your bike into an empty bikebox at any Citybike Wien station. The green light on the bikebox confirmed the successful return of the bike.