This chapter presents an in-depth analysis of harmonic measurements from the Danish 400 kV transmission grid, focusing on methodologies and tools for measuring and analysing harmonic distortions. While appropriate measurements are performed, it is necessary to perform appropriate data processing and analysis. At the beginning of this chapter, the need for harmonic measurements is analysed, as are proper measuring techniques. Afterwards, processed data are analysed by applying Electrometrics, which is a new term introduced in this thesis as “The science of extracting information from electrical systems by means of methods commonly used in data analytical disciplines such as multivariate statistics, applied mathematics, and computer science to solve problems or explore impacts and effects in a power system”. Observations and conclusions from data analyses allow for a better understanding of the nature and origin of variations observed in harmonic measurements in the meshed transmission grid. The application of the results is twofold. These analyses are supposed to support the findings of the propagation method as well as form a basis for understanding harmonic correlations before mitigation strategies are considered.