The Fed’s next big meeting is Tuesday. It last raised interest rates back in December and isn’t expected to do so again this week. But markets will be listening closely to what the Fed has to say about the economy. When global investors literally hang on the Fed’s every word, it’s a tough gig for Fed chair Janet Yellen and company.

“It’s very easy to criticize. It’s very easy to comment,” said Steve Blitz, chief economist at ITG. “But when you’re sitting there, you’re really dealing with a lot of shadows.”

Forget what markets do after the Fed speaks. The full impact of Fed moves on the real economy doesn’t all show up for a while, when the impact on employment and inflation becomes clear.

The challenge for the Fed is figure out just the right time to raise rates, not too soon so as to hurt the recovery, but soon enough to keep things stable. And even though it doesn’t look like that’ll happen this week, the Fed keeps telling us there’s more rate hiking to come this year.