The territorial sea is a strip of sea waters, extending from the coast or internal sea waters of a country in the direction of the open sea, and its outer border is represented by a line whose distance from each point to the nearest point of the starting line is equal to the width of the territorial sea. The rule that every country has its own territorial sea by the very fact that it has a sea coast has grown into an international legal norm, and is part of general international law and is recognized by all relevant international treaties and judgments of international courts. The paper describes and analyzes the space and regime of the territorial sea, the passage of foreign ships through the territorial sea, the destiny of coastal countries in the territorial sea, scientific research and protection against pollution. The territorial sea of Croatia was also analyzed, and a review was given of the problem of delimitation of the territorial sea using the example of Croatia and Slovenia.