Last year's champions -- jockey John Valazquez atop Animal Kingdom -- celebrate winning the 137th Kentucky Derby. The race is known as the "Run for the Roses," after the garland of flowers draped on the winner.
Kentucky (CNN) -- The Kentucky Derby is, without a doubt, America's most iconic horse racing event. England has Royal Ascot, and Australians flock to the Melbourne Cup, but few meetings capture the spirit of a nation quite like the "Run for the Roses."
So named for the garland of roses that is draped over the winner, the Kentucky Derby traces its rich history back to 1875 when Colonel Meriwether Lewis Clark Junior decided to inaugurate a race in his home state of Kentucky.
Clark -- grandson of William Clark of the historic 19th century Lewis and Clark transcontinental expedition -- had been influenced by a visit to England's Epsom Derby. Indeed, the Kentucky Derby was originally run over a distance of 1.5 miles -- the same distance as the Epsom event -- before being shortened to its present 1.25 miles in 1896.
The Kentucky Derby is also the first leg in the venerable Triple Crown, the other two legs being the Preakness Stakes and the Belmont Stakes. Considered one of the sternest tests in thoroughbred racing, only 11 horses have ever completed the treble, the most recent of which was Affirmed back in 1978.
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Union Rags, ridden by jockey Julien Laparoux, is among the hot favorites to win the 138th Kentucky Derby at Churchill Downs.
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Jockey Calvin Borel hopes Take Charge Indy can deliver his fourth Kentucky Derby on Saturday. They have already won March's Florida Derby and will start off among the favorites for the 2012 "Run for the Roses."
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As the years stretch to decades, the search for that elusive champion capable of following in Affirmed's footsteps intensifies. A few have come close: in the last decade War Emblem, Funny Cide, Smarty Jones, Afleet Alex and Big Brown have all come within one victory.
It's a far cry from the 1970s, arguably the races' heyday, when the Triple Crown was won three times -- by Affirmed, Seattle Slew and the great Secretariat.
The enormous chestnut affectionately known as "Big Red" is probably the most famous winner of the Kentucky Derby. Secretariat was the first horse to win the race in under two minutes, and his record of 1:59.40 still stands today.
He became a national celebrity, gracing the covers of Time and Newsweek, following his commanding performances in all three races. His 31-length victory in the Belmont Stakes ranks as one of the finest displays of sporting perfection by any athlete -- human or non-human -- in the 20th century.
Secretariat's dominance in 1973 eclipsed the achievements of one of the sleekest, fastest thoroughbreds ever to grace a racetrack. Had he been born in any other year, Sham would have taken his rightful place among the sport's greats.
His sub-two-minute time in defeat to Secretariat is still the second-fastest Kentucky Derby ever run. But while Secretariat has joined the ranks of the immortals, Sham is a mere footnote in history.
African-Americans have played an integral role in the history of the Derby; indeed, the very first race was won by an African-American jockey (Oliver Lewis) and an African-American trainer (Ansel Williamson).
In an era when African-Americans were still barred from participating in most mainstream sports (it would be more than 60 years before Jackie Robinson made his ground-breaking debut for the Brooklyn Dodgers), African-American jockeys went on to dominate the early years of the Derby, winning 15 of the first 28 races.
That very first Derby was won by Aristides in front of an estimated crowd of 10,000. Nearly 140 years later, more than 100,000 people are expected to flock to Churchill Downs this weekend for a chance to be a part of America's oldest continuous sporting event.
These days, Kentucky shares its Derby with the world. But it remains, in essence, very local in flavor. The hulking Churchill Downs with its famous twin spires is still imposing, but is showing its age. Outside of the Derby and the Oaks (the fillies-only race that takes place the day before the Derby), most of the racing here is low-grade.
Louisville is not a rich town, unlike its well-heeled neighbor Lexington. If the venue feels a little frayed around the edges, that's because racing here is not a glamorous affair.
Once a year, the international racing set descends on Louisville, clogging the skies over the city with private jets. Outrageously-plumed racegoers sip Mint Juleps from air-conditioned boxes while college students on spring break pack the infield. But when the renowned "fastest two minutes in sport" are up, the carnival will move on and racing will return to the people of Louisville.
Kentucky has been the custodian of the world's most emblematic race for more than 100 years, and will continue to preserve its unique traditions for many years to come. It doesn't get more authentic than this.
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Kentucky is the first leg in the Triple Crown, the other two legs being the Preakness Stakes and the Belmont Stakes. Only 11 horses have ever completed the treble, the most recent of which was Affirmed (pictured) in 1978. 
Aristides won in front of an estimated crowd of 10,000. Almost 140 years later, more than 100,000 people are expected to flock to Churchill Downs for a chance to be a part of America's oldest continuous sporting event.
From roses to feathers, the race is just as well known for its fashion as its horses. Once a year, the international racing set descends on Louisville, clogging the skies over the city with private jets. 
While the jetset sip Mint Juleps from air-conditioned boxes, college students on spring break pack the infield.
The Kentucky Derby may be an international event, but it still has a very local flavor. Outside of the Derby and the Oaks -- the fillies-only race that takes place the day before -- most of the racing here is low-grade.
Matz also trained Barbaro, who won the hearts of a nation with his stunning victory at the 132nd Kentucky Derby in May 2006.
Matz looks on as jockey Edgar Prado celebrates Barbaro's commanding seven-length victory.
Hundreds of fans signed a giant "Get Well" card for Barbaro. Although efforts were made to save the colt's leg with pioneering surgery, he eventually had to be put down.
Now aged 61, Matz is hoping to win his second Kentucky Derby on May 5: "To know that you had one horse that could win it and possibly two -- it's a great feeling."
Borel struggled to hold back tears at a press conference after winning the 2009 Kentucky Derby on Mine That Bird in a massive upset result, having come from a long way behind.
Borel celebrates after winning his first Kentucky Derby atop Street Sense in 2007. He was hugged by President George W. Bush at the White House after that victory.
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