Introduction -- Early submersibles -- U-boats in service -- Projects -- Captured submarines -- Manned torpedoes and miniature submarines -- Machinery, sensors and weapons -- The U-boat 'aces' -- U-boat decoys -- U-tankers -- U-boat bunkers -- U-boats in the Mediterranean and Black Sea -- Commercial U-boats : Deutschland and Bremen -- U-boats preserved -- U-boats of the Austro-Hungarian Navy.