We consider security issues in remote state estimation of cyber-physical systems in this chapter. We first investigate the single-sensor case. A sensor node communicates with a remote estimator through a wireless channel that may be jammed by an external attacker. The interactive decision-making process of when to send and when to attack is studied when the sensor and the attacker are both subject to energy constraints. We formulate a game-theoretic framework and prove that the optimal strategies for both sides constitute a Nash equilibrium of this zero-sum game. To tackle the computational complexity issues, we present a constraintrelaxed problem and provide corresponding solutions using Markov chain theory. Under a constraint-relaxed formulation, taking the multisensor data fusion into consideration, the problem for the multiple sensors case is also studied.