In this work, the emergence of polarization and electro-mechanical coupling in Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Nb2/3)O3 – PbTiO3 was investigated by means of thermally stimulated current, and nonlinear dielectric and electro-mechanical measurements. The presence of complex time-dependent and interrelated mechanisms, and their influence on macroscopic functional properties was demonstrated. Widely accepted interpretations related to the nature of relaxor ferroelectrics were questioned. Persistent macroscopic polarization in the nominally ergodic and centrosymmetric relaxor state of pristine Pb(Mg1/3Nb2/3)O3 single crystals was revealed by dynamic pyroelectric measurements. Annealing experiments below and above Burns temperature related this polarization then to the presence of nano-sized polar entities. Poling experiments of Pb(Mg1/3Nb2/3)O3 associated a maximum in the electric-field dependent nonlinear dielectric permittivity and an attendant anomaly in the second harmonic polarization response to the reorientation of nano-sized polar entities. In the following, changes in the polar structure of Pb(Mg1/3Nb2/3)O3 were characterized in-situ with thermally stimulated depolarization current measurements. It was shown that anomalies can be induced by electrical or mechanical poling at temperatures related to static and dynamic nano-sized polar regions. An aging effect was revealed with dynamic dielectric and thermally stimulated current measurements. A current reversal was observed around the coherence temperature in electrical polarized and unpolarized aged samples, suggesting a direct or indirect correlation between oxygen vacancies and nano-sized polar regions. Based on these observations, the effect of aging on macroscopic functional properties was investigated in Pb(Mg1/3Nb2/3)O3 – PbTiO3 by means of thermally stimulated depolarization current, and dielectric and electro-mechanical measurements. For this purpose, the response of unpoled, direct current poled and alternating current poled samples was compared for annealed and aged samples. Aging was related to reduced piezoelectric properties and depolarization temperatures. An apparent contradicting duality of polarization stabilization and property enhancement was observed for alternating current poling. It was proposed that the applied bipolar poling field leads to a distribution of oriented defect clusters throughout the volume, which could improve dielectric and piezoelectric properties through locally broken symmetry and mediated reversible polarization rotation. The existence of an ergodic relaxor state and the model of a single polar entity below TB are not consistent with the results presented in this work. It is proposed that functional and structural properties of undoped Pb(Mg1/3Nb2/3)O3 and undoped Pb(Mg1/3Nb2/3)O3 – PbTiO3 are considerably affected by defect structures, which depend on the thermal, electric field and mechanical field history, and on time. It was speculated that the multitude of sometimes contradicting experimental results reported in the literature are related to these findings. The outcomes of this work are expected to further the fundamental understanding of relaxor ferroelectrics and to contribute to an enhanced reproducibility of experimental results found in the literature.