The Obama administration’s drone program may be Washington’s worst-kept secret, but we still don’t know much about how it works.

From Pakistan to Yemen, strikes are widely reported, and the administration frequently acknowledges them, however evasively. But the program itself remains officially classified, along with the rules and procedures that guide it—if they even exist.

Now that Obama has nominated John Brennan, the chief architect of the targeted-killing program, to run the CIA, everyone has decided it’s time again to try to get some answers. Adding to the new urgency is a leaked memo describing (albeit vaguely) the administration’s legal justification for killing American citizens as well as a report of a secret drone base in Saudi Arabia (even if it turned out to be not so secret after all).

Below, a primer on what we know about the drone program—and what we still need to find out. 

What does the leaked memo tell us?

One might think that a leaked memo would shed light on the rules surrounding the drone program, but in fact the 16-page white paper obtained by NBC raised more questions than it answered. To understand the memo, you’ll have to think back to September 2011, when a drone strike killed Anwar al-Awlaki and Samir Kahn, American citizens who were believed to be working with al Qaeda but who had never been officially indicted. One of the big questions raised by that strike was whether the administration violated the Fifth Amendment (“'No person shall be deprived of life without due process of law”) by killing Awlaki without indicting him or giving him a trial.

The memo explains, sort of, why the administration thought the strike was legal. It says that the government can kill citizens abroad if they poses an “imminent threat,” but it stretches the definition of “imminent” to the point of meaninglessness:

And do targets have to be working with al Qaeda? Apparently not: they can be either a "senior operational leader" of al Qaeda or "an associated force.” And who decides they’re a threat? A “high-level official.” How high-level, how imminent, and what “associated” means are not defined.  

So do we have any idea how strikes get approved?

We know from a report by Newsweek's Daniel Klaidman in May that Obama makes a point of personally approving additions to the “kill list,” the collection of alleged terrorists the administration is targeting. Brennan, the Times reported, is at Obama’s side throughout the process. But other than Obama and Brennan, who can nominate additions to the list? And do Obama and Brennan have any criteria for picking targets, or is it just ad hoc? A New York Times report from several weeks after the election, indicates that they might not have any such rules and had scrambled to develop them when it looked like Mitt Romney might inherit the drones. 

Whom are we killing? 

That’s another big question not answered by the leaked memo. In the early years of the drone program, which began under President Bush in 2002 and was expanded by Obama, the government was targeting senior leaders of al Qaeda believed to be planning attacks on the U.S. (Think back to the seemingly weekly reports that we’d killed al Qaeda’s No. 2.) But as the U.S. killed all those leaders, the strikes continued and began working their way down the totem pole. Now drones are used in Afghanistan, Pakistan, Yemen, and Somalia, frequently against low-level militants engaged in battles with local governments. In some cases, we’re not even sure whom we’re killing. One controversial element of the drone program is the use of “signature strikes”: instead of targeting an individual (a “personality strike”) the CIA targets people whose behaviors match those of militants. The Times recounts a State Department joke that whenever the CIA sees three guys doing jumping jacks, they think it’s a terrorist training camp. 

Could we be killing civilians? 

Another major criticism of the drone program is that its potential for civilian casualties could be turning people against the U.S. and driving them into the arms of al Qaeda. The official line is that civilian casualties, in the words of Brennan, are “exceedingly rare.” One of the advantages of drones is that they can hover overhead for days, waiting to get a clear shot. But we also know that the CIA counts any military-aged male killed by a strike as a combatant. Last August, when a drone strike incinerated three Yemeni al Qaeda members, a moderate cleric who had gone to negotiate with them, and the cleric’s cousin whom he’d brought along for protection, they all would have been initially counted as militants. 

Just how big is the program? 

Again, no one knows exactly, because it’s secret. The Bureau of Investigative Journalism, which compiles public reports of drone strikes, estimates that about 300 strikes have been launched in Pakistan since 2004, between 40 and 50 in Yemen since 2002, and between 3 and 9 in Somalia since 2007. On the upper end, somewhere between 3,000 and 4,500 people have been killed.