Cyber Storm II started this Monday and will end this Friday. It is taking place in Washington DC, and is bringing together federal, state and local governments, the private sector and international participants.
The exercise will simulate a co-ordinated cyberattack on information technology, communications, chemical and transportation systems.
"Securing cyberspace is vital to maintaining America's strategic interests, public safety and economic prosperity," said Greg Garcia, Homeland Security assistant secretary for cybersecurity and communications.
"Exercises like Cyber Storm II help to ensure that the public and private sectors are prepared for an effective response to attacks against our critical systems and networks."
Cyber Storm II will include 18 federal departments and agencies, nine US states, four other countries - Australia, Canada, New Zealand and the UK - and more than 40 private companies. These include ABB, Air Products, Cisco, Dow Chemical Company, Harris Corporation, Juniper Networks, McAfee, Microsoft, NeuStar, PPG Industries and Wachovia.