Who: Christa Davies

Age:  39

Executive Position: Executive Vice President and CFO of Aon

Why she's young and fierce: Davies clocked in at No. 23 on this year's Fortune list of "40 Under 40" of business's hottest rising stars. Prior to joining Aon, she worked as CFO of Microsoft's largest and most profitable sector, the platform and services division. Now, the 39-year-old works as VP and CFO of the world's largest insurance broker.

At Aon, Davies has organized the firm's biggest deals ever, including a $5 billion acquisition of Hewitt Associates this summer, notes Fortune. In addition, last year she helped Aon replace AIG as Manchester United's principal sponsor for the next four years.