The first chapter presents an introduction to three-phased power systems and it mentions some of the basic facts about three-phase systems such as: lower electrical losses, a lower cost of material and the presence of a rotational magnetic field. It also mentions some of the basic connections of three-phase circuits such as the star and delta connections and the relationship between phase and line values of voltage and current. The second chapter focuses generaly on three-phased power systems. It mentions and explains the basic concepts and problems involved with three-phased systems. It also explains the delta and star conections and relationship between their phase and line values of voltage and current. The third chapter focuses on power in three-phased systems, it explains some basic concepts related to three-phased power such as phase shift between current and voltage, power factor and power triangle. It also explains methods of calculating power in three-phased systems and it mentions several methods of measuring power in three-phased systems in various situations. The last chapter focuses on the analysis of power in three-phased systems and it contains calculations, simulations and lab measurements for various situations of symmetrical and nonsymmetrical voltage distributions and real, RL and RLC loads.