On July 13, 2008, New York City’s poverty rate was 18 percent. Twenty-four hours later it had ballooned to 23 percent. How did more than 400,000 New Yorkers become impoverished overnight? The answer is that Mayor Michael Bloomberg adopted a new and more complex—and, he argued, more accurate—measure of poverty than the one the federal government uses. His action reignited a debate in Washington, D.C., and beyond about how America determines who is poor—a debate that many hope will be settled by the U.S. Congress this year.