Much of this book is based on primary material from the U-Boot-Archiv, which has recently been renamed Deutsches U-Boot Museum (Archiv fur Internationale Unterwasserfahrt; German U-Boat Museum - International Submarine Archive) in Altenbruch near Cuxhaven, and I am most grateful to its founder and director, Horst Bredow, for his unflagging support. A high proportion of this primary material is unique to the archive so there is little point in providing detailed references. The records can easily be found by anyone who is prepared to spend time searching for them among the well-classified files.