The law of naval warfare as it existed in 1899 and as it is understood in 1999 exhibits a few similarities but many differences. The fundamental similarity is that the law of naval warfare can be seen, then as now, as consisting primarily of customary international law. The many differences in this law have been caused by the major changes in war at sea and the law of the sea. In 1899 war at sea meant combat primarily by gunfire between surface warships, control of maritime commerce, and shore bombardment. Today, war at sea also involves nuclear-powered aircraft carriers; supersonic aircraft, helicopters and tilt-rotor aircraft; submarines; high-speed patrol craft; ballistic, cruise, and other guided missiles; long-range secure communications for command, control, computers, intelligence, surveillance, and reconnaissance; radar; underwater sound technology; electronic and information warfare; satellites in space; unmanned aerial and undersea vehicles; and stealth and computer technology; as well as expeditionary and amphibious capabilities. Nevertheless, the fundamental role of navies continues to be to establish control at sea or to deny it to the enemy, linking that control to broad political and economic issues ashore. In view of these constants and changes, this article reviews the state of the law of naval warfare at the end of the nineteenth and twentieth centuries and assesses its future prospects.