Germany has commissioned over 1,500 U-boats (German: Unterseeboot) into its various navies from 1906 to the present day. The submarines have usually been designated with a U followed by a number, although World War I coastal submarines and coastal minelaying submarines used the UB and UC prefixes, respectively. When Germany resumed building submarines in the 1930s, the numbering of the submarines was restarted at 1. The renumbering was restarted at 1 a third time when Germany resumed building submarines in the 1960s.

There were some 380 U-boats commissioned into the Kaiserliche Marine in the years before and during World War I. Although the first four German U-boats—U-1, U-2, U-3, and U-4—were commissioned before 1910, all four served in a training capacity during the war. German U-boats used during World War I were divided into three series. The U designation was generally reserved for ocean-going attack torpedo U-boats. The UB designation was used for coastal attack U-boats, while the UC designation was reserved for coastal minelaying U-boats.

U-boats designed primarily for deep water service were designated with a U prefix and numbered up to 167.

The Type U 66 submarines were originally constructed for Austro-Hungary as the U-7 class, but were sold to Germany at the start of WWI.

Coastal attack torpedo U-boats were smaller craft intended for operation closer to land. They were designated with a UB prefix and numbered up to 155.

Coastal minelaying U-boats were smaller vessels intended to mine enemy harbors and approaches. They were designated with a UC prefix and numbered up to 114.

Thirty-five Type UC III submarines were planned, but only 25 were completed. Nine of these (UC-106 to UC-114) were never commissioned and were awarded to the United Kingdom and France and broken up in 1921. Ten of these (UC-80 to UC-89) were broken up at yard

At the outbreak of World War I Germany took charge of a number of submarines under construction in German shipyards for other countries.