At the beginning of WWII, British shipping was attacked by German submarines repeatedly. The Germans often tried to use the element of surprise in their attacks. Some attacks succeeded and others failed. But what does it take to conduct a surprise attack?   The purpose of this thesis is, based on theories dealing with the element of surprise, to examine the validity of the theories in two surprise attacks at the tactical level carried out by the German submarine U 47 in September and October 1939. The theory is explained through five independent variables; unpredictability, change of pace, analysis of the opponent, deception and initiative. The thesis uses case study methodology with a structured, focused comparison approach.   The results indicate that only three of the five variables appeared in both of the cases. The conclusion is that in order to conduct a surprise attack at the tactical level, the attack needs to be permeated with unpredictability, deception and initiative.