In 1990, the United Nations General Assembly proposed a Model Treaty on Extradition, which establishes the principle that in the interest of control of crime, nations should execute bilateral agreements for international extradition of persons charged with indictable offenses. 
As defined by the U.S. Department of Justice, international extradition is the formal process by which a person found in one country is surrendered to another country for trial or punishment. The process is regulated by treaty and conducted between the Federal Government of the United States and the government of a foreign country. 
It differs both from extraordinary rendition in international relations, as well as domestic interstate extradition U.S.). Extraordinary rendition may still involve an overt process by which the person is surrendered to another country by formal but not judicial methods, or may, as used by the George W. Bush Administration, be a secret process.
Article II of the UN Model Treaty defines an indictable offense as "punishable under the laws of both Parties by imprisonment or other deprivation of liberty for a maximum period of at least one/two year(s), or by a more severe penalty." Note that both countries must recognize the criminality of the offense and agree upon a penalty.
Grounds for refusal
The UN treaty establishes both mandatory and optional grounds for refusing extradition by the Requested State (RS). These provisions may conflict with other treaties.
|RS considers the offense political in nature||The person is a citizen of the RS, which claims jurisdiction|
|RS determines that the judgment or prosecution may be based on race, religion, nationality, ethnic origin, political opinions, sex or status||The RS has decided either not to prosecute, or has terminated prosecution for the offense in question|
|Offense is defined under military but not civil law||A prosecution is pending in the RS|
|The RS has issued final judgment on the act in question||The requesting state could impose a death sentence, unless it has agreed to waive that penalty|
|If, under the rules of either state, the person has become immune from prosecution or punishment for any reason, including lapse of time or amnesty;||The offense was committed outside the jurisdiction of either state|
|Extradition would subject the person to torture or cruel and unusual punishment||Under the law of the RS, the offense is assumed to have been committed in whole, or in part, in the RS|
|The judgment of the Requesting State is based on a trial in absentia to which the convicted person has not offered a defense, and will not have the option to have the case retried||The requesting state would use a extraordinary or ad hoc court or tribunal to try or sentence the individual|
|-||In the view of the RS, while considering the nature of the offense and the interests of the requester, determines that in specific case case, extradition would be incompatible with humanitarian considerations in view of age, health or other personal circumstances of that person.|
In extradition, sometimes also called "rendition", a foreign government has to submit a formal extradition request, which the captive has an opportunity to challenge, in the justice system. Normally, an extradition treaty must exist between the countries.
In the United States, extradition requests can be turned down, by the judicial branch, even if the executive branch is in favor of the foreign nation's request. The judiciary can dismiss an extradition request if the charges the foreign government has leveled against the captive are not crimes in the United States. The judicial branch can dismiss an extradition request if the captive has a reasonable fear of facing cruel and unusual punishment if he or she was extradited, or if they had a reasonable fear they would not face a fair trial.
Under the UN model treaty, all requests shall be in writing, through diplomatic channels, to the justice ministry or other appropriate agency. They will document warrants, sentences, and all relevant evidence.