Sea lines of communication (abbreviated as SLOC) is a term describing the primary maritime trade routes between ports. It is generally used in reference to naval operations to ensure that SLOCs are open, or in times of war, to close them.

In World War I and World War II, the Kriegsmarine attempted to close the SLOCs from North America to Great Britain with the use of submarines. The Allies engaged in a defensive campaign employing escort vessels and convoys to keep these SLOCs open. During World War II, the U.S. Navy successfully closed the SLOCs to Japan, strangling the resource-poor island nation.

During the Cold War, Europe would have required resupply and reinforcement from North America. Soviet Navy strategy was to close these SLOCs in order to maximize their numerical superiority on the European continent.