What is power? Who holds the most? These questions are impossible to answer definitively; power is too fluid, and opinions are necessarily subjective. But one thing is certain: Women today are in positions of greater power, in a wider variety of fields, than ever before.

Encarta pays tribute to 10 American women who are shaping the world today. And we're not forgetting the past: We've also compiled a list of 10 American women who changed the world.

Condoleezza Rice, United States secretary of state.
As national security adviser during George W. Bush's first term, Rice soon established herself as a trusted member of the president's inner circle. Her position in Bush's second term, as a wartime secretary of state, will cement her place in the history books.

Sandra Day O'Connor, retired U.S. Supreme Court justice.
O'Connor began her Supreme Court career in 1981 as a strong conservative, but her subsequent move toward the center established her as a wild card on a Court sharply divided between conservatives and liberals. O'Connor was the first woman to serve on the Supreme Court, but was quoted as saying, "The power I exert on the Court depends on the power of my arguments, not on my gender." She retired in January 2006.

Hillary Rodham Clinton, U.S. senator.
Clinton is the only former first lady ever to become a United States senator. She represents New York, the nation's third most populous state, and her prominence has grown greatly in the Democratic Party -- she is currently one of the top two Democratic candidates campaigning to be the party's nominee for the 2008 presidential election.

Margaret "Meg" Whitman, president and chief executive, eBay.
To the fledgling online company, Whitman brought deep "traditional" business experience from corporations including Procter & Gamble, Walt Disney Company, and Hasbro. Since being named president and CEO in 1998, Whitman has grown revenues and solidified eBay's dominance in the fiercely competitive online auction space.

Ruth Bader Ginsburg, U.S. Supreme Court justice.
After graduating from Columbia University at the top of her class, Ginsburg struggled to find a job in the traditionally male legal profession. Her experiences helped to shape a career often devoted to ending institutionalized discrimination against women.

Anne Sweeney, cochair of media networks, the Walt Disney Company; president, Disney-ABC Television Group.
Sweeney achieved a high profile for jump-starting the struggling Disney Channel, quadrupling its subscriber base. Her success at ABC Television has been no less dramatic. Since Sweeney became president in 2004, the previously last-place network has climbed into frequent contention for the top spot in terms of viewership.

Oprah Winfrey, media executive and personality.
When her talk show was picked up for national syndication in 1986, Winfrey made the fateful decision to form her own production company and take over distribution rights, and a media empire was born. Her book club became the most powerful force in the publishing world; the "Oprah effect" instantly placed her choices atop national bestseller lists. In 2004 Forbes estimated her net worth at over $1 billion.

Martha Stewart, media executive and personality.
Stewart built a media empire on her expertise in cooking, gardening, and home improvement. In 1999, Stewart consolidated her expansive television and print enterprises to form Omnimedia, a corporation listed on the New York Stock Exchange. Omnimedia weathered her five-month incarceration in 2004-2005, stemming from a dubious stock trade. Stewart emerged from prison to host new television shows and expand the scope of Omnimedia's ventures. With Stewart's track record of professional evolution--from model to stockbroker to caterer to media executive--expect the next chapter of Stewart's career to be at least as interesting as those that came before.

Julie Louise Gerberding, M.D., M.P.H., director of the Centers for Disease Control and Prevention.
Gerberding has led the Centers for Disease Control and Prevention (CDC) and the Agency for Toxic Substances and Disease Registry (ATSDR) since July 2002. She currently heads the CDC's efforts to prepare for and counter bioterrorism. Prior to her current posts, Gerberding led the CDC's response to the anthrax attacks of 2001 as acting deputy director of the National Center for Infectious Diseases (NCID). She first joined the CDC in 1998 as director of health care quality promotion for the NCID, developing patient safety initiatives and other programs to prevent infections, antimicrobial resistance and medical errors in health care settings.