Many thousands of individual Distributed Denial-of-Service attacks take place each day. While many of these are relatively small, they are sufficient to take unprepared sites offline and threaten connectivity over large regions of the internet. It is in everyone’s interest to take all possible precautions to thwart these damaging DDoS attacks. This paper provides an overview of how this very common form of attack works, what measures can be taken to help eliminate it and pointers to some of the many technical documents that can provide more detail. Also see the video Understanding and Preventing Reflective DDoS Attacks with M3AAWG Senior Technical Advisor Dr. Richard Clayton of Cambridge University explaining reflective DDoS attacks and some of the actions the industry can take to protect against them.