Chapter 4 is devoted to testing and monitoring - the most important procedures required to ensure the efficient operation of the power grid. The grid must be reliable enough to withstand various types of overloads and disturbances, responding adequately to these in real-time. Consequently, a smart transmission system incorporates not only the traditional network but also systems for self-monitoring, self-restoration, adaptation, and forecasting of generation and demand, along with other components that make it “smart”. Innovative solutions in measurement and testing are essential for the effective monitoring of the smart grid, evaluating its advantages over traditional systems, and providing sufficient data for making optimal decisions. Considerable attention is given to modern sensors that meet the testing and monitoring requirements of a smart grid, as well as the technologies used for these procedures. Examples are provided to illustrate their application in monitoring the condition of main network elements and overall grid performance. According to the authors of the textbook, the most comprehensive discussion of these topics can be found in ( Qi H, Shi J, Jianbo Y, Wei Z (2015) Innovative Testingand Measurement Solutions for Smart Grid. Wiley & Sons, p 278; Fraden J (2008) Handbook of modern sensors: physics, design, and applications. Springer, p 590; Ushakov V, Mytnikov A, Rakhmonov I (2023) High-Voltage Equipment of Power Systems. Springer Verlag, p 298;).