Identifying, Understanding, and Analyzing By Steven M. Rinaldi, James P. Peerenboom, and Terrence K. Kelly T he notion that our nation’s critical infrastruc- tures are highly interconnected and mutually dependent in complex ways, both physically and through a host of information and com- munications technologies (so-called “cyber- based systems”), is more than an abstract, theoretical concept. As shown by the 1998 failure of the Gal- axy 4 telecommunications satellite, the prolonged power crisis in California, and many other recent infrastructure disruptions, what happens to one infrastructure can di- rectly and indirectly affect other infrastructures, impact large geographic regions, and send ripples throughout the national and global economy. In the case of the Galaxy 4 failure, the loss of a single tele- communications satellite led to an outage of nearly 90% of all pagers nationwide [1]. From an interdependency per- spective, it also disrupted a variety of banking and financial services, such as credit card purchases and automated teller machine transactions, and threatened key segments of the vital human services network by disrupting communi- cations with doctors and emergency workers. In California, electric power disruptions in early 2001 affected oil and nat- ural gas production, refinery operations, pipeline transport of gasoline and jet fuel within California and to its neighbor- ing states, and the movement of water from northern to cen- tral and southern regions of the state for crop irrigation [2]-[6]. The disruptions also idled key industries, led to bil- lions of dollars of lost productivity, and stressed the entire Western power grid, causing far-reaching security and reli- ability concerns [7]-[10]. December 2001 IEEE Control Systems Magazine 11 0272-1708/01/$10.00©2001IEEE Peerenboom (jpeerenboom@anl.gov) is with Argonne National Laboratory, 9700 S. Cass Ave., Bldg. 900, Argonne, IL 60439, U.S.A. Rinaldi is with the Air Force Quadrennial Defense Review, Washington, D.C. 20330-1670, U.S.A. Kelly is with the Executive Office of the President, Washington, D.C. 20502, U.S.A