Effective cross-examination and rebuttal of an opponent's witness requires advance knowledge of the expert's opinions and the factual bases underlying those opinions. Rule 26(b)(4) creates the framework through which a party may have discovered the facts and opinions of the opponent's expert witness. The rule established four distinct categories of experts and subjects each category to varying degrees of discovery, based on the principle of meeting the needs of both the party seeking discovery and the party employing the expert. The rule specifies a two-step process that controls discovery of experts expected to testify at trial. Limited immunity is provided to nontestifying experts. The rule also addresses the discovery of information from informal consultants and inhouse experts and of expert information not acquired in anticipation of litigation. Discussion of Federal court decisions and 596 footnotes.