At a time when the plumbing of the financial system is in the spotlight, the youngest division leader at the New York Fed owns the biggest wrench. Appointed earlier this year to head the markets group, the MIT economics Ph.D. and former forecaster manages the Fed's $2.1 trillion portfolio and gives advice to policymakers in D.C. It's a high-profile gig: His predecessor, William Dudley, now heads the New York Fed.

Home front: When he relocated to Larchmont, N.Y., Sack immediately ruled out Tim Geithner's home, still for sale there.

influential, interesting, 40 under 40 (2009),