As Internet is increasingly being used in almost every aspect of our lives, it is becoming a critical resource whose disruption can be very damaging. Denial of service (DoS) attacks in the Internet aim at blocking the availability of computer systems or services. There are various technical and non-technical challenges that need to be well understood, in order to design DoS attack defense solutions that fundamentally address the problem. A comprehensive study that provides a clear analysis of the DoS problem and the solution space can be a tremendous help to security researchers and professionals. This book presents an in-depth study of the DoS problem in the Internet, and aim to provide a clear understanding of (1) the denial of service attack, (2) the targets and possible avenues of carrying out a DoS attack, (3) the challenges of defending against DoS attacks, (4) what has been done in terms of addressing these challenges and countering the various attacks , and (5) what problems still remain to be further investigated.