In this book a new and original approach to the methodology of the design and optimization of three-phase distribution transformers is developed and explained. Two optimization techniques in the MATLAB environment, Global Search - GS and Pattern Search - PS, for minimizing the objective function are applied. By implementing nine optimization variables, the sufficient accuracy of calculations is assured. Involvement of additional constraint functions in the optimization procedure, contributes to achieve the best solution faster then usual. When generating the 3D transformer models it is defined exact magnetizing characteristic of the core material, instead of using software library of magnetic materials.The magnetizing characteristic is approximated by a polynomial of the third order, matching perfectly to the real one. In addition, when running transient 3D FEM analysis it is defined and introduced the Building Factor, which contributes to more accurate computation of iron losses; the fact that is proved by the corresponding measurements of the studied objects