Yesterday, Alberto Gonzales's former chief of staff, D. Kyle Sampson, voluntarily testified before the Senate Judiciary Committee. But because the White House is claiming executive privilege, it is not clear whether, and under what circumstances, committee members will have a chance to question Karl Rove, Harriet Miers, Gonzales, or other top-level political appointees, or to obtain documents from White House staff that bear on the controversy.

What, exactly, is executive privilege, and how can it be used? The following is a brief primer on the existing law of executive privilege, as that law has been developed by the federal courts. (It does not trace the practices of Congress and the executive branch over time.) One recurring problem in executive-legislative conflicts is that much of the law remains unsettled. The Supreme Court has not said a great deal; the court of appeals for the District of Columbia Circuit has said more; but it is not at all clear that a majority of the Supreme Court would agree with what the lower court has said.

The simplest lessons are as follows:

•  Executive privilege applies to White House communications, including conversations involving the president or among his closest aides as part of an effort to inform a direct presidential decision.

•  These communications are presumed to be privileged and generally need not be disclosed.

•  Congress or courts might, however, require disclosure if there is specific reason to believe that malfeasance or wrongdoing has occurred and if the relevant information cannot be obtained elsewhere.

•  For Congress, an especially important question is whether access to the relevant information is necessary for the performance of functions that involve a) oversight and b) lawmaking itself.

•  Large, unanswered questions are whether the strong forms of the privilege apply to a) conversations not involving the president or b) communications with respect to a decision that is not made by the president personally.

Here is (the bulk of) the law as it now stands:

1. The president has a generalized privilege against disclosure of his own discussions with his close advisers.

2. This privilege is not absolute. The Supreme Court ruled in 1974 in United States v. Nixon that the "generalized interest in confidentiality" can be overcome by the need for evidence that "is demonstrably relevant" to a criminal trial, if that evidence is "specific and central to the fair adjudication in a particular criminal case."

3. The Court also said, however, that the privilege might be absolute if the case involves "a claim of need to protect military, diplomatic, or sensitive national security secrets." (Note: Points 2 and 3 apply in either a grand jury proceeding or a criminal trial.)

4. The Court's 2004 decision in Cheney v. The United States District Court further established that the privilege apparently applies to the vice president no less than to the president, at least where the communications of the vice president are intended to culminate in advice to the president.

5. That ruling suggests that, in a civil case, it will be harder to overcome the privilege, because civil cases do "not share the urgency or significance of the criminal subpoena." A generalized request for information, in a civil case, will not overcome the privilege.

6. How does executive privilege apply to conflicts between Congress and the president? The Supreme Court has not said. The key rulings come from the D.C. Circuit, and the Supreme Court might not agree with that court. (See points 12 and 13 below.)

7. To whom does executive privilege apply? The Supreme Court has not clearly said. One view is that the privilege applies only to communications involving the president. Another view holds that the privilege applies to everyone within the executive branch. The D.C. Circuit has ruled that the privilege applies to presidential advisers in the course of preparing advice for the President for decisions that he will directly make. The privilege therefore includes a) communications authored by such advisers and b) communications which these advisers solicited and received from others.

8. But the presidential communications privilege "should be construed as narrowly as is consistent with ensuring that the confidentiality of the President's decision-making process is adequately protected." This means that the privilege does not extend "to staff outside the White House in executive branch agencies." The privilege is limited to "communications authored or solicited and received by members of an immediate White House adviser's staff who have broad and significant responsibility for investigating and formulating the advice to be given to the President on the particular matter to which the communications relate." Under this ruling, the privilege does not extend to internal Justice Department documents that are not sent to the president, but it is not at all clear whether the Supreme Court would agree with this view.

9. Direct decision-making by the president is required. If the president himself is not directly involved, there is no privilege. Thus the court of appeals has said that the attorney general and the deputy attorney general cannot "be equated with the close presidential advisers" protected by the privilege.

10. The appointment and removal power are quintessential and nondelegable, and hence communications related to the exercise of that power are privileged.

11. The court of appeals has said that propositions 7-10 might not bear on the scope of the privilege in the congressional-executive context, because the "President's ability to withhold information from Congress implicates different constitutional considerations."

12. According to the court of appeals in 1974, Congress' ability to obtain information from the president should be analyzed under the United States v. Nixon framework. This means that a generalized request from Congress, unaccompanied by "particularized showings" of need, will run into trouble.

13. It is not enough for a Senate committee to overcome the privilege by saying that it needs materials "in order to resolve particular conflicts in the voluminous testimony it has heard, conflicts relating to 'the extent of malfeasance in the executive branch,' and, most importantly, the possible involvement of the President himself." At least this is so--the court of appeals said in 1974--when another committee had the relevant materials and when these materials could not be shown to be "critical to the performance of ... legislative functions." But this is a very narrow ruling in light of presidential release of many materials in that case and the "subsequent and on-going investigation of the House Judiciary Committee" with respect to impeachment.

14. The executive branch has a generalized "deliberative process privilege," rooted in the common law. The privilege covers "documents and other materials that would reveal 'advisory opinions, recommendations and deliberations comprising part of a process by which governmental decisions and policies are formulated.'" This privilege applies throughout the executive branch.

15. The deliberative process privilege is highly qualified and it depends on a balancing test; it is far weaker than the presidential communications privilege. When "there is reason to believe that the documents may shed light on government misconduct, 'the privilege is routinely denied.'" "The deliberative process privilege is a qualified privilege and can be overcome by a sufficient showing of need. This need determination is to be made flexibly on a case-by-case, ad hoc basis." Courts will take into account such factors as "the relevance of the evidence," "the availability of other evidence," "the seriousness of the litigation," "the role of the government," and the "possibility of future timidity by government employees."

16. It is possible that conflicts between Congress and the president in this domain would be taken to present a political question and hence not subject to judicial resolution at all--but unlikely, because this view has attracted close to no support on any federal court.

17. What, in particular, does United States v. Nixon mean by a demonstrated, specific need, sufficient to overcome the presidential communications privilege? The court of appeals has said that a party seeking to overcome the privilege must demonstrate 1) "that each discrete group of the subpoenaed material likely contains important evidence" and 2) "that this evidence is not available with due diligence elsewhere."

18. Questions without clear answers therefore include a) how far down the line the privilege extends, b) the relationship of United States v. Nixon to conflicts between Congress and the president, c) whether the latter conflicts might present a political question, d) the nature, basis, and scope of the deliberative process privilege, and e) the precise showing that must be made to overcome the privilege. With respect to e), a generalized interest in possible wrongdoing is far less likely to overcome the privilege than a specific need to answer the question whether specifically described wrongdoing has occurred. With respect to b), a key question is whether Congress needs the relevant material in order to perform its constitutionally specified functions, above all lawmaking.

Correction: This article originally omitted the tenth point in the above summary. We regret the error.

By Cass R. Sunstein