A power transformer is one of the most responsible and capital-intensive elements of the energy system. The main tasks of ensuring reliable and uninterrupted operation of transformers were defined at the session of CIGRE 2012. Among those was the elaboration of new diagnostic methods. The article presents data on the state of the transformers park in power systems around the world. Recently, one has witnessed the intensive development of transformer equipment diagnostic methods. The article describes the trends in selecting the most effective methods to assess the equipment technical condition without disconnecting it from a network. Besides, the article reviews the problem of determining a parameter set necessary and sufficient for the most comprehensive assessment of the transformer technical state. In the energy systems a large number of diagnostic methods and systems are developed and used. The article shows that the solution of diagnostic problems is primarily associated with the use of complex systems, development and implementation of online diagnostics methods which use the latest achievements in the field of information-measuring systems. Among all the methods of online diagnostics much attention has currently been paid to the electromagnetic control methods. The article shows that the most informative method is based on the analysis of natural electromagnetic radiation (EMR) spectra. Benefits and features of the presented method are described by the example of 500 kV transformers control. The criteria and methods of transformer equipment assessment are recommended. The results agree well with the test data obtained by other methods.