This paper explains the concept of thermal imaging, a method that has recently been increasingly used in the maintenance of power plants. The second chapter contains basic concepts from heat and describes the thermodynamic foundations of heat transfer, among which thermal radiation is highlighted, which is of great importance in thermal imaging. The third chapter describes the parts of a thermal imaging camera, which represents the central object of a thermal imaging system, and presents the basic classification of cameras. The fourth chapter discusses various areas of application of thermal imaging, with an emphasis on its application in the power system. The fifth chapter presents basic concepts and classifications of distribution substations that connect medium-voltage and low-voltage networks. The sixth chapter presents images and results of thermal imaging inspections carried out on several 10/0, 4 kV substations. By reviewing the thermograms shown, results on the current state of the substations were obtained and recommendations for their maintenance were given.