The analysis of data represents the starting point for many applications in the power systems, either in the design phase, either in operation for on-line control. The need to extend the capabilities of human analysis for to handle the overwhelming quantity of data that we are able to collect has become vital. The ability of human operators to manually analyse large databases is far exceeded. Since computers have enabled the possibility of storing large amounts of data, it is natural to resort to computational techniques to help us to discover meaningful patterns and linkages between variables. This book is essentially devoted to using of the clustering techniques in optimal operation and planning of power systems. Numerous problems are treated and clustering based solutions are proposed. These problems are referred at: profiling load and consumers' classification, peak load estimation, power/energy losses estimation, analyse of slow voltage variations, distributed generation, the assessment of the renewable energy potential, coordination between distribution generation sources, human errors monitoring, voltage control, and optimal placement of phase measurements units.