While inheriting a billion dollars is still the easiest way to land on our list of the world's wealthiest, it certainly isn't the most common. Almost two-thirds of the world's 946 billionaires made their fortunes from scratch, relying on grit and determination, and not good genes.

Fifty of these self-made tycoons are college or high school dropouts. The most famous billionaire dropout is Microsoft's Bill Gates, who finally got his honorary degree from Harvard University in June, 30 years after quitting the prestigious school to sell software. ''I did the best of everyone who failed,'' joked the world's richest man in his official graduation address. With failure like that, who needs success?

Other billionaires, such as media maven Oprah Winfrey, made their fortunes against far greater odds. Born in rural Mississippi, she spent her early years living in poverty on her grandmother's farm. Wanting a way out, she moved to Wisconsin to be with her mother, but was sexually molested by her male relatives. At age 14, she reportedly gave birth to a premature baby who died. Only after moving to Nashville to be with her father did her luck finally start to turn.

• Slideshow: Billionaire rags to riches
• In pictures: World's billionaires 2007

In honor of the world's self-made billionaires, we're recounting 10 of our favorite real-life Horatio Alger tales.

The stories of these bootstrapping billionaires are as diverse as the 10 individuals themselves. They range in age from 40 to 91, hail from diverse industries such as fashion and oil, and live in five different countries. Russia's richest man, Roman Abramovich, was an orphan. Apple's iconic Steve Jobs was adopted. Jobs dropped out of Reed College when he couldn't pay the tuition; his net worth today could support nearly 40,000 students at Reed for four years. Three others, including Ralph Lauren, are also college dropouts.

Another five are high school or grade school dropouts, proving that street smarts can often trump book smarts. The U.K.'s publishing magnate Richard Desmond, for instance, quit high school when he realised he could make more money working in the cloakroom of a club; at age 16, he borrowed his older brother's suit to get a sales job. He's been selling ever since, peddling music, porn and celebrity titles including OK! magazine.

In pictures:

• New billionaires
• Billionaire bachelors

Asia's richest man, Li Ka-shing dropped out of school at age 15, after his father died, to work in a factory. Kirk Kerkorian quit during the eighth grade to take up boxing. He later flew airplanes on daredevil missions across the Atlantic during World War II, before sinking his money into his own airline and reinvesting profits in Las Vegas.

Sin City has also been good to Sheldon Adelson. The son of a Boston cabdriver borrowed $200 at age 12 to start selling newspapers; he later held stints as a mortgage broker, investment advisor and financial consultant. The high school dropout and Broadway enthusiast studied voice in his teens, but it was another kind of stage that called him--trade shows, where he made his first fortune.

Adelson later gambled on casinos in Las Vegas, Macau and Singapore, and took his Las Vegas Sands public in December 2004. Says Adelson, ''I loved being the outsider.''

In pictures:

• Billionaire women we envy
• Young billionaires

Good luck and good timing is also helpful when creating vast fortunes from scratch. James Cayne, for instance, moved to New York to play bridge full-time; he was spotted by Wall Street legend Alan "Ace" Greenberg, who was impressed by Cayne's card skills and hired him to be a stockbroker at his firm Bear Stearns. Cayne is now chairman.

The world's wealthiest novelist, J.K. Rowling, was on welfare raising her little girl when her agent called to tell her that Bloomsbury would publish her book about an adolescent wizard named Harry Potter.

One thing is for sure: There is no lack of chutzpah among our rags to riches bunch.