The Six Days of Chicago was a six-day cycling event, held in Chicago, Illinois between 1915 and 1957. It took place fifty times during that period, as two editions were often held in one year. Gustav Kilian holds the record for most wins with a total of six, between 1935 and 1939.

Six-day cycling is a track cycling event that takes place over six days. Six-day races started in Britain, spread to many regions of the world, were brought to their modern style in the United States and are now mainly a European event. Initially, individuals competed alone, the winner being the individual who completed the most laps. However, the format was changed to allow teams, one rider racing while the other rested. The 24-hours a day regime has also been relaxed, so that most six-day races involve six nights of racing, typically from 6pm to 2am, on indoor tracks (velodromes). Six-day events are annually hosted in London, Berlin, Ghent, Copenhagen, Hong Kong, Manchester, Melbourne and Brisbane.