The relaxor ferroelectric Pb(Mg_1/3 Nb_2/3 )O₃ [PMN] has been extensively studied in the last years. A short overview is presented on various physical models and microstructural features of PMN to understand the peculiar properties of this material. Some important aspects related to the electric field-induced first-order phase transition, from the average cubic phase to a macroscopically ferroelectric 3m phase, are discussed on the basis of an E-T phase diagram. The induced ferroelectric phase transition is characterized by an intrinsic Curie temperature Tco=213 K associated with a threshold field strength Eth=1.75 KV/cm, and a tricritical point at which the first-order transition changes into a second-order. The relationship is discussed between the compositionally non-stoichiometric nanoclusters and the induced polar macrodomain. The mechanism of the induced phase transition can be described as a transformation from nanodomain states to macrodomain states with long-range symmetry breaking, revealing the induced ferroelectric nature of PMN.