This chapter presents the “Source-Field” method, which is a very effective way to solve 3D magnetostatic and magnetodynamic problems. An interesting point related to the following presentation is the use of the tree and co-tree technique, which is an appropriate way to assemble and properly solve the linear systems generated by the Source-Field method. In fact, applying the tree technique, the solution of the resulting linear equation system is free of troubles when solved by either direct or iterative algorithms. Permanent magnets were and are nowadays often used in electrical devices. The magnetic induction values are higher with the scalar potential. It is reflected in the obtained results. For the magnetic energy and with scalar potential, it is around 14.70% higher than the energy calculated by the vector potential technique.