Piracy is of note in international law as it is commonly held to represent the earliest invocation of the concept of universal jurisdiction. The crime of piracy is considered jus cogens, a conventional peremptory international norm from which states may not derogate.

Those committing thefts on the high seas, inhibiting trade, and endangering maritime communication were considered by sovereign states to be hostes humani generis (enemies of humanity).

In international law the right of search denotes the right of a warship to detain and search a private vessel belonging to a foreign national.  In peacetime, this right is ordinarily exercised only within the territorial waters (see waters, territorial) of a state and merely as an incident of the power to police such waters, and generally only in such cases as suspected piracy, violation of fishing regulations, or interference with telephone cables. Since piracy often takes place outside the territorial waters of any state, the prosecution of pirates by sovereign states represents a complex legal situation. To prosecute pirates on the high seas, states most derrogate from the conventional freedom of the high seas, and violate the principle extra territorium jus dicenti impune non paretur, that being: the judgment of one who is exceeding his territorial jurisdiction is disobeyed with impunity.

However, as jus cogens, jurisdiction can typically be exercised against pirates without objection from the flag state of the pirate vessel. Since piracy is a crime against humanity, those practicing it may be tried in any competent court, regardless of nationality. To the forms of piracy defined by international law, however, a nation may add offenses committed on board its own vessels or in its own territorial waters. 

As the line between privateering and piracy is often hard to draw, any act of doubtful legality committed on the seas is apt to be characterized as piracy. Thus the sinking of merchant vessels by the Germans in World War I was characterized by some as piracy, although the act was done on the authority of a national state. However, at the Washington Conference of 1921 a treaty was concluded that declared that improper visit and search by one in the service of any power would constitute piracy.

In wartime, a belligerent may search neutral vessels on the high seas in order to capture the property of enemy nationals or to remove contraband bound for enemy ports. Forcible resistance to search allows the warship to attack or destroy the vessel or its cargo or to take them as a prize.