Much has been said in the press about immunity from prosecution without explanation or clarification of its use or application. Generally speaking, as reported in the font of all knowledge, Wikipedia:
“Immunity from prosecution is a doctrine of international law that allows an accused to avoid prosecution for criminal offenses. Immunities are of two types. The first is functional immunity, or immunity ratione materiae. This is an immunity granted to people who perform certain functions of state. The second is personal immunity, or immunity ratione personae. This is an immunity granted to certain officials because of the office they hold, rather than in relation to the act they have committed.”

Functional immunity, recognized in both international law and treaty law confers immunity upon persons performing acts of state. An example: foreign officials who in the course of performing an act of state commit a criminal act such as a hit and run while driving in the course of government business. Once out of office, the immunity may not continue, as reflected in the arrest in the United Kingdom of Chile’s ex-president Pinochet for violation of the United Nations Convention against Torture as well as several prosecutions in the International Criminal Court* located in The Hague, Netherlands.

Personal immunity generally applies to heads of state and senior government officials during their time in office. Clause 1, Section 6 of the U.S. Constitution states in part for members of Congress,

They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.

In the U.S. there is a practice of witness immunity whereby a prosecutor essentially agrees never to prosecute the crime that the witness might have committed in exchange for evidence. Any grant of immunity impairs the witness’s right to invoke the Fifth Amendment protection against self‑incrimination as a legal basis for refusing to testify, both at trial as well in Grand Jury proceedings. Refusal to testify, when granted immunity, may subject the witness to the penalties of contempt of court including possibly incarceration.

Witness immunity may be granted in one of two forms: Transactional immunity or Use and Derivative Use immunity. Transactional immunity, colloquially known as “blanket” or “total” immunity, completely protects the witness from future prosecution for crimes related to his or her testimony. Use immunity and derivative use immunity prevents the prosecution from using the witness’s own testimony or any evidence derived from his testimony against him. It is generally used in order to compel a witness to testify, often in high profile organized crime litigation. However, a prosecutor is free to use other evidence that it may have independently obtained against the individual who was given use immunity and derivative use immunity to force testimony.

There has been some controversy regarding the consequences of blanket grants of immunity by congress as effectively derailing criminal prosecutions, suggesting that such grants violate the separation of powers doctrine. One example in recent history was the immunity granted to Oliver North that resulted in the failure of the Justice Department’s prosecution for his role in the Iran-Contra scandal. With congress seeking to undertake its various investigations and requests for extensive testimony the issue is likely to continue to arise.

*The United States government has consistently opposed this court due to its concern about possible charges against US nationals.