Presidents can assert executive privilege to prevent government employees from sharing information.

It was the second time Bush asserted executive privilege in four days.

That means any executive privilege claim has likely been waived, Stern said.

With Obama's asserting executive privilege, however, that's become a lot less likely.

The judge would then decide the merits of an executive privilege claim.

The White House could respond to committee demands by citing executive privilege.

Typically a president uses executive privilege to prevent government employees from releasing information.

But there are instances of executive privilege being challenged in court.

Obama asserted executive privilege to block the disclosure of the documents.

Could Trump be challenged in court if he asserts executive privilege?

Much like the attorney-client privilege, executive privilege is intended to keep conversations private.

Here is a look at some notable invocations of executive privilege.

Nixon withheld tape recordings and other material subpoenaed by Jaworski, citing executive privilege.

The White House has asserted executive privilege over the documents withheld by Holder.

Legal experts said Trump's executive privilege claim would be weak.

That litigation would lead to a ruling on whether Trump lawfully invoked executive privilege.