A letter rogatory is a formal request from a court in one country to "the appropriate judicial authorities" in another country requesting compulsion of testimony . documentary or other evidence, or effect of service of process.
Letters rogatory can be used in civil and criminal matters, and have been used in administrative matters. The execution of a request for judicial assistance by the foreign court is based on comity between nations, absent a specific treaty obligation such as the Hague Evidence Convention or Mutual Legal Assistance in Criminal Matters (MLAT) treaties. Consular Conventions generally include language which authorizes transmission of letters rogatory through diplomatic channels. This does not obligate the foreign country to execute the request, but simply provides a formal avenue by which the requests may be made. If there is no consular convention in force between the United States and the foreign country, letters rogatory are received by foreign authorities on the basis of comity. Letters rogatory are a time consuming, cumbersome process and should not be utilized unless there are no other options available.
The foreign court will execute a letter rogatory in accordance with the laws and regulations of the foreign country. In obtaining evidence, for example, in most cases an American attorney will not be permitted to participate in such a proceeding. Occasionally a local, foreign attorney may be permitted to attend such a proceeding and even to put forth additional questions to the witness. The power of federal courts to issue letters rogatory derives from 28 U.S.C. 1781 and from the court's "inherent" authority.
A letter rogatory can also be called a letter of request, rogatory letter or requisitory letter.