(FORTUNE Magazine) – AMERICA'S LEADING businesswomen are calling the shots more than ever before. And that means they're making some tough choices--pushing a nervy acquisition, say, or in the case of the powerful woman on the cover, revamping her executive team. It's a testament to how far women have risen in the corporate world that the story is what they are doing when they make it to the corner office, not just that they happen to be there.

In this, FORTUNE's eighth annual Most Powerful Women issue, we pull back the curtain on this new world. We profile Martha Stewart and her comeback strategy. We let six women explain a difficult choice they made on the job; we ask whether women make decisions differently from men; we explore the dearth of women in hedge funds; and we talk to women who have reached the heights--then chosen to walk away.

Our Most Powerful Women lists required some tough choices too. We considered four criteria: the size and importance of the woman's business; her clout; her career trajectory; and the cultural and social impact. A dozen who made it in 2004 missed the cut this year--a sign that the bar keeps getting higher. In the pages that follow, you'll be dazzled by corporate stars with talent and moxie, and not a man among them. That only means their roads are less traveled--and all the more compelling. -- Jia Lynn Yang