In the author's model, these 20 international crimes are aggression; war crimes; unlawful use of weapons; crimes against humanity; genocide; apartheid; slavery and slave-related practices; torture; unlawful medical experimentation; piracy; hijacking; kidnapping of diplomats; taking of civilian hostages; unlawful use of the mails; drug offenses, falsification and counterfeiting; theft of archeological and national treasures; bribery of public officials; interference with submarine cables; and international traffic in obscene publications. This categorization is based on an act being designated as a crime in a multilateral convention that reflects specific penal characteristics and is recognized by a significant number of states. The author outlines these penal criteria and then discusses inconsistencies among international conventions, individual development patterns for the identified international crimes, and the evolution of language in conventions. A discussion of enforcement schemes addresses international tribunals and the debate over the explicit or implicit duty to prosecute or extradite in cases involving international crimes. The paper includes 35 footnotes.