We have characterized the dynamics of the polar nanoregions in Pb (Mg _1/3 Nb_2/3)O₃ through high-resolution neutron-backscattering and spin-echo measurements of the diffuse-scattering cross section. We find that the diffuse-scattering intensity consists of both static and dynamic components. The static component first appears at the Curie temperature Θ∼400 K while the dynamic component freezes completely at the temperature T_f ∼200 K; together, these components account for all of the observed spectral weight contributing to the diffuse-scattering cross section. The integrated intensity of the dynamic component peaks near the temperature at which the frequency-dependent dielectric constant reaches a maximum (T_max) when measured at 1 GHz, i.e., on a time scale of ∼1 ns. Our neutron-scattering results can thus be directly related to dielectric and infrared measurements of the polar nanoregions. Finally, the global temperature dependence of the diffuse scattering can be understood in terms of just two temperature scales, which is consistent with random-field models.