It is a legacy at once triumphant, compelling and clouded by skepticism.

Armstrong, who has said he will retire from competitive cycling after this year's Tour, stamped himself as one of the greatest cyclists ever -- surely the greatest American cyclist ever -- by stringing together six victories in the sport's premier event. His success and celebrity have given cycling the kind of widespread, magazine-cover attention unimaginable in the United States only six years ago, when his epic streak began.

Off the bike, Armstrong's impact on the fight against cancer is similarly staggering. First he conquered the disease -- he was diagnosed with testicular cancer in 1996, and tumors spread to his lungs and brain -- and now the Lance Armstrong Foundation pours millions of dollars into cancer research and patient support. His yellow LiveStrong wristbands have become ubiquitous -- more than 50 million have been sold (at $1 each) since their introduction 14 months ago.

Armstrong's legacy also includes lingering suspicions in a sport long known for its culture of performance-enhancing drug use. He has never tested positive for banned drugs and repeatedly has denied using them, but a former masseuse and former personal assistant have leveled doping allegations, and one company, spurred by those accusations, challenged its obligation to pay Armstrong a $5 million bonus for his sixth Tour win.

Against this multi-layered backdrop, he now hops on his bike one final time, hoping to achieve a rare feat in sports: walking away at the height of dominance. Armstrong, 33, insists this is his farewell, even if other athletes who towered over their sports the way he does -- such as Michael Jordan in basketball -- could not resist the temptation of a comeback.

Even if he falls s