A threat is a circumstance, condition, or event with the potential to cause harm to personnel and/or network resources in the form of destruction, disclosure, modification of data, denial of service, and/or fraud, waste, and abuse. This chapter describes the most common security threats to network systems. Network security threats include impersonation, eavesdropping, denial of service, packet replay, and packet modification. For a more detailed presentation addressed to a technical specialist with in-depth knowledge of network protocols, see [CB94].