Al Qaeda leader Osama bin Laden warned his associates about drone strikes.

The CIA flew the stealthy UAV RQ-170 over bin Laden's compound in Pakistan to monitor it in advance of the raid that killed him, according to robotics warfare expert Peter Singer.

The United States has 8,000 drones. The U.S. Army has a robust plan for using them more and more in the future.

U.S. officials recently signed a deal with Niger to house surveillance drones in that country to keep tabs on Islamic militants in the region.

How many drone strikes has the U.S. carried out?

The New America Foundation estimates, based on news reports, that the U.S. government has carried out 349 "CIA drone strikes" in Pakistan and 61 in Yemen. The foundation is a Washington-based, non-partisan think tank.

The United States does not release figures on the number of strikes. President Obama surprised many people in January 2012 by officially acknowledging that the attacks even exist.

In the midst of a Google + video chat, he said "a lot of these strikes" have been in Pakistan's Federally Administered Tribal Areas, along the border with Afghanistan, where many members of al Qaeda and the Taliban are known to be. "For us to be able to get them in another way would involve probably a lot more intrusive military actions than the one we're already engaging in," the president said.

Who has been killed by drone strikes?

The New America Foundation estimates that in Pakistan, between 1,953 and 3,279 people have been killed since 2004 -- and that between 18% and 23% of them were not militants. The "non-militant casualty rate" was down to about 10% in 2012, the group says.