The practical aspects of maritime security consist of many elements including port security measures,vessel monitoring, intelligence collection, and cooperation. One aspect of maritime securitythat is an essential element, which regardless of other developments can never be entirely dispensedwith, is the interdiction and boarding of vessels. There will always be a necessity on the part of governmentalvessels to intercept, board, and bring under control ships at sea. This chapter considers therelevant international law to the interdiction and boarding of vessels at sea, including some recentdevelopments designed to widen the circumstances where a boarding might take place.The law of the sea has traditionally not been sympathetic to measures toward the interdiction ofvessels other than that of the flag state, except in extremely limited circumstances. The grundnormpoint is the basic principle of noninterference with vessels at sea, unless they are flying your state'sflag or have engaged in behavior giving rise to universal jurisdiction, such as piracy or the slavetrade. This has been the situation for many years, as is borne out by a statement by Lord Stowellalmost two centuries ago:In places where no local authority exists, where the subjects of all States meet upon a footing of entireequality and independence, no one State, or any of its subjects, has a right to assume or exerciseauthority over the subjects of another. No nation can exercise a right of visitation and search upon thecommon and unappropriated parts of the sea, save only on the belligerent claim.Even if a jurisdictional basis can be found, international law circumscribes the means available tocompel a vessel to comply with an order to heave to. This chapter also explores the relevant authoritydealing with the use of force at sea during peacetime to indicate what range of action can be usedby a commander to compel compliance.