U-boat is the Americanized version of the German word U-Boot, a shortening of Unterseeboot, literally "undersea boat". While the German term refers to any submarine, the English one (in common with several other languages) refers specifically to military submarines operated by Germany, particularly in the First and Second world wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role (commerce raiding), enforcing a naval blockade against enemy shipping. The primary targets of the U-boat campaigns in both wars were the merchant convoys bringing supplies from Canada, the British Empire, and the United States to the islands of the United Kingdom and (during the Second World War) to the Soviet Union and the Allied territories in the Mediterranean. Austro-Hungarian navy submarines of the First World War (and before it) were also known as U-boats. The first submarine built in Germany was the three-man submarine Brandtaucher, which sank to the bottom of Kiel harbor during its first test dive. The vessel was designed in 1850 by the inventor and engineer Wilhelm Bauer and built by Schweffel & Howaldt in Kiel for the Imperial German Navy. Brandtaucher was later rediscovered during dredging operations in 1887, and raised sixteen years later and placed in a museum in Germany.

This was followed in 1890 by WW1 and WW2, built to a Nordenfelt design. In 1903, Germaniawerft dockyard in Kiel completed Germany's first fully functional submarine, Forelle which was sold to Russia during the Russo-Japanese War in April 1904. The SM U-1 was a completely redesigned Karp-class submarine and only one was built. It was commissioned by the Imperial German Navy on 14 December 1906. It had a double hull, was powered by a Körting kerosene engine and was armed with a single torpedo tube. The fifty percent larger SM U-2 had two torpedo tubes. A diesel engine was not installed in a German navy boat until the U-19 class of 1912–13. At the start of World War I, Germany had 48 submarines of 13 classes in service or under construction. During the First World War the SM U-1 was used for training and was retired in 1919. It is on display at the Deutsches Museum in Munich.