The attorney–client privilege is the strongest legal privilege. It is limited to communications between attorney and client made as part of the attorney–client relationship. To be legally privileged, a communication must pass directly from one party to the other, and it must pass intentionally. It may be written, spoken, signed, or otherwise communicated. It extends only to the communication itself, it cannot be used to hide information that was not previously privileged. The privilege only prevents the court from forcing the attorney to testify. For example, if the client tells the same information to a friend as to the attorney, the friend may be forced to testify about the information. The privilege cannot be used to hide documents that were not prepared by the attorney, and it does not extend to physical evidence.

This example from criminal law will help illustrate the nature of a communication. One of the traditional privileges in criminal law is the protection of communications between husband and wife. This privilege is intended to preserve domestic relations. It prevents people from testifying about information they were told by their spouse. If a husband is told by his wife that she has been filing fraudulent Medicare claims, he may not testify if she is prosecuted for criminal Medicare fraud. However, if he actually watches her filling out fraudulent forms on their home computer, he may be compelled to testify as to his observations.

The only exception to the attorney–client privilege is the threat of future harm. The attorney may alert the police if the attorney believes the client is going to commit a crime that would endanger others. The state may also require attorneys to report child abuse and other public health problems, but few states do.