Distributed Denial of Service (DDoS) attacks is virulent, relatively new type of attacks on the availability of Internet services and resources. DDoS attackers infiltrate large number of computers by exploiting software vulnerabilities, to set up DDoS attack networks. DDoS attacks pose an immense threat to the Internet, therefore some defense systems are required to defend these attacks. In this paper, an overview of DDoS problem, brief detail of most recent DDoS incidents on online organizations is highlighted. Finally three approaches to evaluate DDoS defense systems i.e theory, simulation and emulation are discussed. Keywords— DDoS, Incidents, availability, emulation, vulnerability.