A subpoena (/səˈpiːnə/; also subpœna or supenna) or witness summons is a writ issued by a government agency, most often a court, to compel testimony by a witness or production of evidence under a penalty for failure. There are two common types of subpoena:
The term subpoena is from the Middle English suppena and the Latin phrase sub poena meaning "under penalty". It is also spelled "subpena". The subpoena has its source in English common law and it is now used almost with universal application throughout the English common law world. John Waltham, Bishop of Salisbury, is said to have created the writ of subpoena in the reign of Richard II. However, for civil proceedings in England and Wales, it is now described as a witness summons, as part of reforms to replace Latin terms with Plain English understandable to the layman.
In New South Wales, a court may set aside the whole, or part of, a subpoena on the basis that it is a "fishing expedition". In Lowery v Insurance Australia Ltd, the NSW Court of Appeal held that where documents requested in the schedule of a subpoena are deemed to have no relevance to the proceedings in dispute, the subpoena may be set aside as it has no legitimate forensic purpose. It was also held that it was not the role of the Court to redraft the subpoena and narrow its scope to those issues in dispute. In Victoria a subpoena is usually issued by a court registry officer, and does not require leave of the court.
In New Zealand, subpoenas are governed under the rules of the court in which the subpoena is issued.
Subpoenas are usually issued by the clerk of the court in the name of the judge presiding over the case. Additionally, court rules may permit lawyers to issue subpoenas themselves in their capacity as officers of the court. Typically subpoenas are issued "in blank" and it is the responsibility of the lawyer representing the plaintiff or defendant on whose behalf the testimony is to be given to serve the subpoena on the witness. If a witness is reluctant to testify, then the personal service of subpoena is usually required with proof of service by non-party server.
The subpoena will usually be on the letterhead of the court where the case is filed, name the parties to the case, and be addressed by name to the person whose testimony is being sought. It will contain the language "You are hereby commanded to report in person to the clerk of this court" or similar, describing the specific location, scheduled date and time of the appearance. Some issuing jurisdictions include an admonishment advising the subject of the criminal penalty for failure to comply with a subpoena, and reminding him or her not to leave the court facilities until excused by a competent authority. In some situations the person having to testify or produce documents is paid.
Pro se litigants who represent themselves, unlike lawyers, must ask a court clerk to officially issue them subpoena forms when they need to call witnesses by phone or in person, or when they need to officially request documents to be sent to them and/or directly to court. Any documents that have not been subpoenaed to court or verified by a witness may be dismissed by the opposite party as hearsay, unless excepted by hearsay rules or permitted by the judge. If the witness is called via long-distance phone call, then the requesting party is responsible for initiating the call and providing a payment with a prepaid phone card. Most states (including California) have further restrictions on subpoena use in criminal cases. 
Some states (as is the case in Florida) require the subpoenaing party to first file a Notice of Intent to Serve Subpoena, or a Notice of Production from Non-Party ten days prior to issuing the subpoena, so that the other party may have ample time to file any objections.
Also, the party being subpoenaed has the right to object to the issuance of the subpoena, if it is for an improper purpose, such as subpoenaing records that have no relevance to the proceedings, or subpoenaing persons who would have no evidence to present, or subpoenaing records or testimony that is confidential or privileged.
Standing committees in both houses of the United States Congress have the authority to send out subpoenas for legitimate lawmaking and investigation purposes. This compels the production of testimony or records, and failure to respond constitutes contempt of Congress.