The world's most powerful billionaires

The authority and influence Michael Bloomberg wields is staggering.

As mayor of New York City--America's largest and most complex metropolis--he lords over more than 8 million people speaking 40 different languages within 305 square miles. He commands 311,000 city employees and an annual budget of $60 billion.

Since being elected in 2001, Bloomberg has resurrected New York from the wreckage of the 9/11 attacks, seized control of the city's schools, enforced a ban on smoking in bars and restaurants, and taken serious strides to make the city a leader in energy efficiency.

On any given day, he might negotiate a workers strike, coordinate various law-enforcement agencies' efforts to combat terrorist threats or handle another crisis (like, say, a commercial airplane crashing into the Hudson River).

And he paid for this job. Bloomberg left his massive media and financial information company, Bloomberg LP, in 2001 to effectively buy City Hall when he chipped in $74 million of his own cash to run for mayor. (He threw in another $85 million to keep the gig four years later and recently convinced New York's City Council to allow him to run for a third term this year.)

While he's been mayor, Bloomberg LP has grown. Today, the firm has 10,000 employees in 126 global offices, providing most of the world's trading floors with financial data. Bloomberg's 88% stake in the company helped push his net worth to $20 billion last fall, making him one of America's richest men.

This combination of wealth, media influence and political prowess lofted Bloomberg atop the annual Forbes list of the most powerful billionaires in the world.

Last March, there were 1,125 billionaires in the world, each wielding tremendous wealth and weight over the markets and industries in which they operate. But few plutocrats possess the money, economic dominance and political clout to touch--or the potential to touch--all of us.

To compile the list, Forbes reporters created a formula based on the size and scope of the industries billionaires control, the political influence they exert and the fortunes they hold.

Behind Bloomberg is Italian Prime Minister Silvio Berlusconi, who heads a nation of 58 million people, a diversified industrial economy with a gross domestic product of $2.4 trillion and a military budget of roughly $43 billion.

Berlusconi's company Fininvest is a player in life insurance, movie production and sports teams. It also controls much of the Italian television market.

As the worldwide recession deepens and industries look to governments for help, a prime minister can have serious sway over the stock market. Shares of Italian automaker Fiat (nyse: FIA - news - people ) soared 6% earlier this month after Berlusconi pledged aid to the Italian auto industry.

Billionaires with political power are rivaled by those who control portions of the commodities markets. Indian industrialist Lakshimi Mittal controls 10% of the world's steel production through his company ArcelorMittal ; despite his fortune falling $24.5 billion between March and November 2008, he ranks third on our list.

Other moguls lord over portfolios of companies. Warren Buffett, through holding company Berkshire Hathaway, controls more than 50 companies that had a combined $118 billion in sales in 2008. He ranks fifth.

Throughout the credit crisis, Buffett injected much-needed capital into ailing companies. His $5 billion investment in Goldman Sachs  last September caused shares to leap 6%. He pumped billions more into General Electric and Swiss Re. Even President Obama has turned to Buffett for economic advice.

Perhaps no billionaire has more control over how America spends its disposable cash than Oprah Winfrey. Her daily talk show airs in 141 countries and reaches more than 46 million viewers. With Oprah's approval, an unknown book instantly turns into a bestseller, and getting a product endorsed on Oprah's Favorite Things show can be the crowning achievement of a marketer's career.

According to research compiled by two University of Maryland economics professors, Oprah's endorsement of Barack Obama lent the candidate an estimated additional one million votes in the 2008 Democratic presidential primary.