In her speech, which focused on labor markets, Yellen said the Great Recession complicated the Fed's ability to assess the U.S. job market and made it harder to determine when to adjust interest rates. Yellen offered no signal that she's altered her view that the economy still needs support from the Fed in the form of ultra-low interest rates.
The timing of a Fed rate increase remains unclear, however most investors expect the first one to come sometime in 2015.
INVESTORS' INTEREST IN INTEREST RATES: The nation's key interest rate, known as the Federal Funds Rate, has been near zero since late 2008 in order to simulate economic activity and demand. But the economic downside to low interest rates is the possibility that the U.S. economy could recover too quickly, heat up, and cause inflation.
The Fed Funds rate helps determine interest rates on a variety of financial services including mortgages, credit cards and the yields that bond pay. Many investors believe the U.S. economy has recovered enough from the depths of the financial crisis to warrant higher interest rates.
The Fed has been winding down another economic stimulus program, large-scale purchases of bonds in the open market, since December.
DRAGHI'S TURN: The Jackson Hole meeting's other big speech will be given by European Central Bank chief Mario Draghi. He's expected to give his view on the eurozone economy, which is stagnating even as rates are ultralow and stimulus is plentiful. "Overall, the tale of two speeches today is likely to be that monetary policy stimulus is still here for a while yet in the U.S., and a very long while in Europe," said Rabobank analyst Michael Every.
UKRAINE: International markets turned lower earlier in the day after a Russian convoy of aid entered Ukraine, defying the government there. The trucks are meant to bring supplies to residents in rebel-held zones where separatists are fighting with the Ukrainian government. There are concerns the move by Moscow could cause a direct confrontation between Ukrainian and Russian units. Germany's DAX fell 0.4 percent and France's CAC 40 fell 0.5 percent.
ELECTRIFYING: Dynergy jumped $4.28, or 14 percent, to $33.96 after the company announced it was buying $6.25 billion in power plants from Duke Energy and Energy Capital Partners. The deal would double Dynergy's power generation capabilities. Duke Energy rose 68 cents, or 1 percent, to $73.72 on the news as well.
ENERGY: Benchmark crude oil for October delivery fell 32 cents to $93.64 in New York. The contract rose 51 cents on Thursday.
AP Business Writer Kelvin Chan contributed to this report from Hong Kong.