In American civil procedure, the work-product doctrine protects materials prepared in anticipation of litigation from discovery by opposing counsel.
[3] Under the work-product doctrine, "tangible material or its intangible equivalent" that is collected or prepared in anticipation of litigation is not discoverable.
Additionally, it includes materials collected for the attorney such as interrogatories, signed statements, other information acquired for the prosecution or defense of a case.
"Memoranda, briefs, communications ... other writings prepared by counsel for his/her own use in prosecuting the client's case ... mental impressions, conclusions, opinions, or legal theories" are never discoverable by an opposing party.
[6] The work-product doctrine originated in the 1947 case of Hickman v. Taylor, in which the Supreme Court affirmed a United States Court of Appeals for the Third Circuit decision which excluded from discovery of oral and written statements made by witnesses to a defendant's attorney.