The North Sea is a marginal, epeiric sea of the Atlantic Ocean on the European continental shelf. It is more than 600 miles long and 350 miles wide, with an area of around 222,000 square miles. A large part of the European drainage basin empties into the North Sea including water from the Baltic Sea. The North Sea connects with the rest of the Atlantic through the Dover Strait and the English Channel in the south and through the Norwegian Sea in the north.

The North Sea averages about 100 m (325 ft) deep, with a maximum depth of 700 m (2300 ft) and in some areas shallows can be a mere 15 m deep. The North Sea lies above what used to be the triple junction between three continental tectonic plates in the early Paleozoic Era. Movement on the faults associated with these tectonic phenomena can still cause earthquakes and small tsunamis. The sea's coastal features are the result of glacial movements rather than tectonics. Deep fjords and sheer cliffs mark the coastline of the northern part of the North Sea, whereas the southern coasts consist of sandy beaches and mudflats. These flatter areas are particularly susceptible to flooding, especially as a result of storm tides. Elaborate systems of dikes have been constructed to protect coastal areas.

The development of European civilization has been heavily affected by the maritime traffic on the North Sea. The Romans and the Vikings sought to extend their territory across the sea. Both the Hanseatic League and the Netherlands sought to dominate commerce on the North Sea and through it to access the markets of the world. Britain's development as a sea power depended heavily upon its dominance in the North Sea, where some of its rivals sought power, first the Netherlands and finally Germany and to a lesser extent Russia and the Scandinavian nations. Commercial enterprises, growing populations, and limited resources gave the nations on the North Sea the desire to control or access the North Sea for their own commercial, military, and colonial ends.

Its importance has turned from the military to the economic. Traditional economic activities, such as fishing and shipping, have continued to grow and other resources, such as fossil fuels and wind energy, have been discovered and developed.