Argus, a framework for defending a public utility against cyber-physical attacks, contains intelligent checkers that use invariants derived from the physical and chemical interactions among various components and products of a utility. An Argus implementation is independent of the traditional layered defense that employs firewalls and other network-based logic to prevent intrusions into control systems, and hence is referred to as orthogonal defense. Portions of Argus have been implemented and tested in an operational water treatment testbed and found effective in detecting a number of single and complex multicomponent deception attacks.