The technological progress connects physical plants with the cyber-world tighter than ever before. The modern challenge is to protect such a cyber-physical system (CPS) against cyber-attacks. This chapter shows the danger of a new type of stealthy cyber-attack strategy called mirror attack. If the adversary gains access to the communication network, he may cut off the connection between the plant and the controller, replace the control input signals calculated by the controller by his attack signals, and take over the control of the physical plant. Simultaneously, the adversary makes use of the plant model to generate fake sensor output signals to replace the true sensor output signals of the plant. In this way, the adversary keeps the mirror attack stealthy while manipulating the plant. Different from covert attacks, a mirror attack can remain stealthy even in the case of actuator saturation. The mirror attack has also more freedom than replay attacks. Moreover, it is shown that, even if adversarial knowledge of the plant model is not accurate, the robust mirror attack may still remain stealthy. Experimental results on a laboratory three-tank system demonstrate the stealthiness of the mirror attack and the robust mirror attack.