A Flag State is the country to which a ship is legally linked in terms of nationality, usually following registration proceedings. This link is evidenced by the flag that the ship is obliged to fly.
A flag State exercises its right to enforce regulations and control the activities of ships flying its flag (ie. its authority in terms of legislation, jurisdiction and policing, as well as its right to carry out checks in administrative, technical and social matters). The UNCLOS states that the flag State shall effectively exercise its jurisdiction  and shall, in particular, compel ships flying its flag to observe its national law, as well as rules laid down by international law to which the State adheres. There is, however, no superior authority liable to oblige the State to exercise its jurisdicton (cf. Sovereignty).
The flag State's jurisdiction is exercised outside the bounds of those granted to coastal States in areas in which the latter exercise their jurisdiction (internal waters, territorial seas, the continental shelf and Exclusive Economic Zones). In principle, the flag State's jurisdiction is of an exclusive nature on the high sea, to the extent that no entity exercises jurisdiction there superior to that of said State .
On the high sea, this exclusive jurisdiction implies that, in principle, no other State can check on, or apply sanctions to, a foreign vessel in order to oblige it to comply with pertinent rules of law. This exclusiveness is liable to raise difficulties regarding compliance with international regulations pertaining to the preservation of resources or protection of the marine environment. These difficulties may be of a material kind (in terms of organizing the investigations), but also of a legal nature, in that international regulations are not always applicable to the flag State (cf. Opposability).
Material difficulties in carrying out investigations may be overcome on condition that the flag State and States disposing of the means of conducting investigations on the high sea adopt an agreement offering such States the authority to carry out investigations on vessels flying the flags of States party to the agreement (cf. Effectiveness of the law). Difficulties of a legal kind relating to the inopposability of an international rule to a State can only be overcome by means of diplomacy, by getting the relevant flag State to agree to observe the international rules in question (cf. Conditions of a State's commitment). Finally, another way of circumventing difficulties posed by the flag State's exclusive jurisdiction on the high sea is to resort to the jurisdiction of the Port State.
 It should be mentioned that there are a few exceptions to this principle: the most noteworthy concerns the ancestral struggle against acts of piracy (an exception which, however, cannot be applied in the fields of fishing or the fight against pollution).