Swimming has featured in every edition of the Games since 1896. Early Olympic events were conducted in freestyle (crawl) or breaststroke. Backstroke was added as of 1904. In the 1940s, breaststrokers discovered they could go much faster by bringing both arms overhead together. This was soon banned in the breaststroke, but became the butterfly stroke, which is now the fourth stroke used in competitive swimming.
Men and women now compete in 17 events each, involving four different strokes across a range of distances. Freestyle races cover 50m, 100m, 200m, 400m, 800m (women only), 1500m (men only) and 10km. The butterfly, backstroke and breaststroke races each cover 100m and 200m. All four strokes are used in the 200m and 400m individual medley events. The 4 x 100m freestyle, 4 x 200m freestyle and 4 x 100m medley relays complete the program.
Each race has a maximum of eight swimmers. Preliminary heats in the 50m, 100m and 200m lead to semi-finals and finals based on the fastest times. In relays and individual events of 400m or more, the eight fastest finishers in the preliminaries advance directly to the finals.