Written or oral materials prepared by or for an attorney in the course of legal representation, especially in preparation for litigation. Generally, an opposing party may not discover or compel disclosure of work product. In limited circumstances, however, an opposing party may discover or compel disclosure of work product upon a showing of "substantial need" and "undue hardship." Fed. R. Evid. 26(b)(3).
See, e.g. Hickman v. Taylor, 329 U.S. 495 (1947).