Surprise occurs when someone is unaware of an attack coming and unable to respond to it. This normally involves combat. If a character is attacked from behind, he is surprised. If a character is attacked from Unseen Presence (or Vanish from the Minds Eye), he is surprised. If the target character has Heightened Senses (or Telepathy), there are certain exceptions - please refer to the Auspex discipline for more information. Someone who is surprised may not harm the attacker, even if he wins the challenge. In the case of physical combat you may only defend with Stamina based physical Traits and cannot burn blood to give bonus physical Traits, though you can still heal if you are harmed, as this takes place after being hit. You cannot use a Brawl, Melee or Dodge Re-Test as you are not actually fighting, just trying to absorb the damage without harm.
This is an advanced rule from within The Challenge System.