The coming decades may see the large scale deployment of networked cyber–physical systems to address global needs in areas such as energy, water, health care, and transportation. However, as recent events have shown, such systems are vulnerable to cyber attacks. We begin by revisiting classical linear systems theory, developed in more innocent times, from a security-conscious, even paranoid, viewpoint. Then we present a general technique, called "dynamic watermarking," for detecting any sort of malicious activity in networked systems of sensors and actuators. We present an experimental demonstration of this technique on a laboratory version of a prototypical intelligent transportation system, and a simulation study of defense against an attack on Automatic Gain Control (AGC) in a synthetic four area power system. [Joint work with Bharadwaj Satchidanandan, Woo Hyun Ko, Le Xie and Tong Huang].