There are 35 Olympic sports to be played at this year's Summer Games in London.

We all know about swimming, track and field, and gymnastics. But the inclusion of some other sports has raised some controversy.

Critics have questioned the addition of badminton or curling, but at least they're aware of them as Olympic sports.

But then there's handball, trampoline and racewalking too.

So just how does a sport become a designated Olympic event? According to the International Olympic Committee's website, it must be administered by an international federation, which ensures its compliance with the Olympic charter.

If the sport is widely practiced around the world and meets the criteria established by the IOC, the organization's program commission may recommend that the sport be included in the games.

Trampolining: It technically falls under the gymnastics category. Rosannagh MacLennan of Canada won the gold medal in women's trampoline on Saturday, while Dong Dong of China captured the gold in the men's competition Friday.

According to the website London2012.com, the first modern trampoline was initially used as a training tool for tumblers, astronauts and athletes.

Trampolining became increasingly popular. The first Trampoline World Championships were held at the Royal Albert Hall in London in 1964.

Trampoline became an Olympic sport at the Summer Games in Sydney in 2000.

Racewalking: Yes, it's an actual Olympic event that falls under the umbrella of athletics. Men compete in 20 km and 50 km races, while women compete in a 20 km race.

For these athletes, racewalking is very serious business. Russian racewalker Valeriy Borchin, who was the reigning Olympic gold medalist in the sport, collapsed Saturday during the final stretch of the men's 20 km walk across the city of London. Chen Ding of China went on to the win take the gold medal.

American Trevor Barron, 19, finished 26th in the event. At 1 hour, 22 minutes and 46 seconds, he had the fastest time ever clocked by an American in the Olympic event.

According to USA Track and Field, racewalking differs from running in that participants must have one foot firmly on the ground at all times, so the athlete's back toe may not be lifted until the heel of the front foot is on the ground. Also, the front leg must be straightened when it hits the ground. Failure to comply with the rules results in penalties that can lead to disqualification from the event.

There's also a World Cup of racewalking. That event is held every two years.

Handball: This is a fast-paced team game that was first introduced to the Olympics in 1936. Two teams of seven players compete by passing, throwing, catching and dribbling a small ball with their hands while trying to score goals.

The team with the most goals wins the game. A game consists of two 30-minute halves with a 10-minute half-time break.

The reigning women's champion is the Norwegian team, while the reigning men's champions are French.

Hammer Throw: Hammer throw is a field event in which participants attempt to hurl a hammer - a heavy metal ball attached to a wire cable - as far as they possibly can. The athlete who throws the hammer the farthest wins the competition.

The sport is played by men and women. The athlete starts in a throwing circle, with the head of the hammer allowed to rest on the ground. Wearing a glove, the athlete rotates in the circle up to four times before releasing the hammer.

Krisztian Pars of Hungary took the gold medal in the sport on Sunday, throwing the hammer weighing about 16 pounds more than 264 feet.

Oksana Menkova took the women's gold medal in hammer throw at the 2008 Summer Games in Beijing. The women's competition starts this week. Women throw a hammer weighing just less than 9 pounds.

Fencing: According to the official website of the International Olympic Committee, fencing began the move from a form of military swordplay training to a sport in either the 14th or 15th century.

The subsequent evolution of the sport - to include wire-mesh masks and a sword with a flat tip - led to its increasing popularity, according to the websdite.

Men's fencing appeared at the Olympics in Athens in 1896, with a women's event being included in 1924 at the Paris games.

Today, men and women compete in team and individual events, using three types of weapons: the foil, epee and saber.