Butterfly is the most physically demanding of all the strokes

Swimming has always been one of the most thrilling and eagerly anticipated sports at the Olympics.

It is the focus of the first week of any Olympiad and in Athens that will be no different despite a fraught build-up to the event.

Earlier in the year plans to put a roof on the Aquatic Centre were controversially abandoned. But when the action starts, despite the searing temperatures, all the focus will be on the action in the water.

Men and women compete in 16 events each, an identical programme apart from the men's 1500m freestyle and the women's 800m freestyle.

Freestyle - commonly known as front crawl down at your local baths - is the fastest stroke on show, with breaststroke the slowest. Swimmers also compete in backstroke and butterfly.

• Freestyle
  Not commonly known is the fact that freestyle is not actually a stroke. In this event swimmers can pick whatever stroke they wish, but the reality is they always pick the fastest one, the crawl.

  DID YOU KNOW? In freestyle and backstroke, swimmers can use any part of their body to touch the end of the pool

  During the freestyle event, some part of the body must be above the surface of the water at all times, apart from 15 metres after a start or a turn when competitors may swim underwater.

• Backstroke
  As the name suggests, competitors must swim on their backs with their eyes focused on the ceiling, although they can rotate their body to some degree as they swim.

  This is the only race where competitors start in the water although they can still spend 15 metres underwater from the gun and after a turn.

• Breaststroke
  Swimmers must swim face down moving their arms and legs together in a horizontal direction and while their head can be immersed completely, it must break the surface of the water during every complete stroke.

  While underwater at the start and the turn, swimmers are only allowed to make one arm stroke and one leg kick and they must touch the end of the pool with both hands.

• Butterfly
  The most physically demanding of all the strokes evolved out of a loophole in the breaststroke laws.

  It differs in that the arms and legs, which must move together during a stroke, move vertically rather than horizontally.

EVENTS Freestyle: 50m, 100m, 200m, 400m, 800m (women only) & 1500m (men only) Backstroke: 100m & 200m Breaststroke: 100m & 200m Butterfly: 100m & 200m Individual medley: 200m & 400m Relay: 4x100m free, 4x200m free & 4x100m medley

In the medley relay a different swimmer swims each leg using a particular stoke, but in a different order; backstroke, breaststroke, butterfly and freestyle.

Each race has a maximum of eight swimmers. There are preliminary heats in the 50m, 100m and 200m distances that lead to semi-finals and then finals, all based on who gains the fastest times.

In the relays and individual events the eight fastest finishers in the preliminary heats advance straight through to the finals.

Only two swimmers per country, per individual event, are allowed to compete.