Boardroom battles and power plays shaped this year’s U.S. list, which boasts eight new faces and – surprise! – a new No. 1.

By Beth Kowitt & Rupali Arora

There’s been plenty of turmoil atop Fortune’s annual Most Powerful Women list. Meg Whitman crashed the party, coming in at No. 9 when she became CEO of Hewlett-Packard HPQ . (As CEO of eBay EBAY , she was on the list from 1999 to 2007.) Longtime MPW Carol Bartz lost her job as CEO of Yahoo YHOO — and her place on the list — while Oprah Winfrey fell 10 spots to No. 16, her power and influence in flux without the platform of her eponymous syndicated talk show. Perhaps the biggest change of all? Kraft KFT CEO Irene Rosenfeld takes the No. 1 position from PepsiCo PEP chief Indra Nooyi, who topped the list for five years. This ranking is all about power, and while Nooyi runs the bigger company, Rosenfeld’s decision to split Kraft into two entities shows she has it and knows how to use it.

But for all these high-profile changes, there are many more women making big moves with little fanfare. Wal-Mart’s HPQ Rosalind Brewer (No. 23) has an important new job, running all of the retailer’s stores in the East. In addition to Whitman, there are four other females who became CEOs of Fortune 500 companies since our last power ranking. That brings the total number of women CEOs in the Fortune 500 to 15 — up from just two on our list when it debuted in 1998.

50 Most Powerful Women: The Full List

This article is from the October 17, 2011 issue of Fortune.