Attorney–client privilege or lawyer–client privilege is the common law doctrine of legal professional privilege in the United States. Attorney–client privilege is "[a] client's right to refuse to disclose and to prevent any other person from disclosing confidential communications between the client and the attorney."

The attorney–client privilege is one of the oldest privileges for confidential communications. The United States Supreme Court has stated that by assuring confidentiality, the privilege encourages clients to make "full and frank" disclosures to their attorneys, who are then better able to provide candid advice and effective representation.

The origins of attorney–client privilege trace back to medieval England, where the king presided over trials and relied on attorneys to present cases. Because attorneys were considered officers of the court, they were expected to fully disclose all relevant information. However, as legal representation evolved, courts recognized that forcing attorneys to reveal client confidences undermined justice. This led to a principle that even the king could not compel an attorney to disclose privileged communications.

One of the earliest recorded cases affirming this privilege is Berd v. Lovelace (1577), where an English court ruled that legal counsel could not be forced to testify about client communications. By the 18th century, the principle had solidified in English common law, emphasizing that the privilege belonged to the client, not the attorney. This doctrine carried over to the American legal system, where it became a foundational rule of professional ethics. The U.S. Supreme Court has repeatedly affirmed its importance, notably in Upjohn Co. v. United States, 449 U.S. 383 (1981), which broadened the privilege to cover corporate legal communications.

With respect to experts that are hired by the attorneys, the attorney-client privilege is referred to as a Kovel standard based on the case of United States v. Kovel, 296 F.2d 918 (2d Cir. 1961) or broadly a Kovel Agreement. Experts hired by attorneys to assist in representation of a client may vary by profession. Such experts include (but are not limited to) CPAs, actuaries, medical doctors, or engineers. These experts may be disclosed or undisclosed to the court. In the United States disclosed Expert witnesses may not be covered under the Kovel Standard, depending on the court and the nature of their work, and their involvement in the legal advice process.

Although there are minor variations, the elements necessary to establish the attorney–client privilege generally are:

There are a number of exceptions to the privilege in most jurisdictions, chief among them:

A corollary to the attorney–client privilege is the joint defense privilege, which is also called the common interest rule. The common interest rule "serves to protect the confidentiality of communications passing from one party to another party where a joint defense or strategy has been decided upon and undertaken by the parties and their respective counsel."