This paper presents the foundations of control and estimation over lossy networks. The authors analyze the impact of packet loss and delay on feedback loops in control systems, and propose a generalization of classical control theory to account for the stochastic nature of communication channels. The paper discusses the differences between TCP-like and UDP-like protocols, where the former provides acknowledgments of successful packet delivery, and the latter does not. For TCP-like protocols, the separation principle holds, and the optimal LQG controller is a linear function of the estimated state. In contrast, for UDP-like protocols, the optimal controller is generally nonlinear. The paper also presents the theoretical foundations for optimal estimation and control under both protocols, including the derivation of optimal estimators and controllers, and the analysis of their stability under different conditions. The results show that the stability of the system depends on the arrival probabilities of packets, and that the optimal controller can fail to stabilize the system if these probabilities fall below certain thresholds. The paper concludes with the development of necessary and sufficient conditions for closed-loop stability under both TCP-like and UDP-like protocols.This paper presents the foundations of control and estimation over lossy networks. The authors analyze the impact of packet loss and delay on feedback loops in control systems, and propose a generalization of classical control theory to account for the stochastic nature of communication channels. The paper discusses the differences between TCP-like and UDP-like protocols, where the former provides acknowledgments of successful packet delivery, and the latter does not. For TCP-like protocols, the separation principle holds, and the optimal LQG controller is a linear function of the estimated state. In contrast, for UDP-like protocols, the optimal controller is generally nonlinear. The paper also presents the theoretical foundations for optimal estimation and control under both protocols, including the derivation of optimal estimators and controllers, and the analysis of their stability under different conditions. The results show that the stability of the system depends on the arrival probabilities of packets, and that the optimal controller can fail to stabilize the system if these probabilities fall below certain thresholds. The paper concludes with the development of necessary and sufficient conditions for closed-loop stability under both TCP-like and UDP-like protocols.