In naval warfare, the line of battle is a tactic in which the ships of the fleet form a line. It originated with the navy of the Commonwealth of England and perhaps with General at Sea Robert Blake who wrote the Sailing and Fighting Instructions of 1653. The tactic was used by both sides in the Anglo-Dutch Wars with Michiel de Ruyter of the United Provinces being the best early exponent.

The line of battle has the advantage over previous naval tactics — in which ships closed on each other for individual combat — that each ship in the line can fire its broadside without fear of hitting a friendly ship. A ship powerful enough to stand in the line of battle came to be known as a "ship of the line" (of battle) or a "line of battle" ship which shortened to become "battleship". Line of battle tactics continued to be used (for example in the Battle of Jutland in 1916 and the Battle of Surigao Strait in 1944) until the end of World War II.

World War II saw the decline of line of battle tactics, as admirals quickly realized that with the long (and lengthening) arm of aircraft carriers changing the strategic calculus, lining battleships up to pound away at each other had become more of a liability than a strength, as carrier fighters could swoop down and pounce on battleship formations easily. Battle group tactics became the norm as World War II progressed, with a single capital ship (carrier or battleship) and its escorts became a fleet unto itself.

After World War II, the carrier battle group became the accepted paradigm of strategic naval formations.