Attorney-client privilege is an evidentiary rule that protects communications between a client and his or her attorney and keeps those communications confidential. It protects both attorneys and their clients from being compelled to disclose confidential communications between them made for the purpose of furnishing or obtaining legal advice or assistance. The privilege is designed to foster frank, open, and uninhibited discourse between attorney and client so that the client's legal needs are competently addressed by a fully prepared attorney who is cognizant of all the relevant information the client can provide. The attorney-client privilege may be raised during any type of legal proceeding, civil, criminal, or administrative, and at any time during those proceedings, pre-trial, during trial, or post-trial.
In United States v. United Shoe Machinery Corp., 89 F. Supp. 357 (D. Mass. 1950) the court articulated five requirements necessary to establish attorney client privilege. They are first, the person asserting the privilege must be a client, or must have sought to become a client at the time of disclosure; second, the person connected to the communication must be acting as a lawyer; third, the communication must be between the lawyer and the client exclusively — no non-clients may be included in the communication; fourth, the communication must be for the purpose of securing a legal opinion, legal services, or assistance in some legal proceeding, and not for the purpose of committing a crime; fifth, the privilege may be claimed or waived by the client only (usually, as mentioned, through counsel).
However, even when all the requirements have been met, the courts can still compel disclosure of the information sought. The courts base exceptions to the privilege on rule 501 of the Federal Rules of Evidence, which states that "the recognition of a privilege based on a confidential relationship … should be determined on a case-by-case basis." In examining the privilege on a case-by-case basis, the courts weigh the benefits to be gained by upholding the privilege (preserving the confidence between attorney and client) against the harms that may be caused if they deny it (the loss of information valuable to the opposing party).
The attorney-client privilege is considered as one of the strongest privileges available under law.