A substantial area of commercial activity, maritime law covers transport of people and cargo from one place to another aboard ships. It is subject to extensive official intervention, with the result that a number of bodies form a genuine maritime administration that extends well beyond national borders. It also includes the bodies responsible for the safety of ships and their passengers, as well as the policing of the high seas.

The main legal framework is UNCLOS, but it can be supplemented by more specific international conventions and the customary law of states. The laws of the sea have a global character, although it is increasingly common for disputes involving the law of the high seas to be resolved at the International Court of Justice (ICJ) in The Hague.

Admiralty law has been given practical expression in the flag principle, under which ships are subject to the laws of the state whose flag they fly. The practical application of this principle is accomplished through registration, which is not just a simple licensing procedure but invests the state with responsibilities and duties regarding the ship’s internal operation and its compliance with international law.

Banks which loan money to purchase ships, vendors who supply them with necessaries like fuel and stores, seamen who are owed wages, and other creditors often have liens upon a ship that can be enforced by arrest or seizure. These are maritime liens, and in the United States they may be brought in either federal or state courts. Ordinary mortgages securing property in a ship or its gear or cargo do not qualify as maritime liens. The decision of whether an operation is law enforcement or military under Article 2(4) of UNCLOS relies on a broad assessment of the circumstances surrounding each case.