[en] Random lattice disorder produced by chemical substitution in ABO₃ perovskites can lead to the formation of dipolar impurities and defects that have a profound influence on the static and dynamic properties of these materials that are the prototypical soft ferroelectric (FE) mode systems. In these highly polarizable host lattices, dipolar entities form polar nanodomains whose size is determined by the dipolar correlation length, r_c, of the host and that exhibit dielectric relaxation in an applied ac field. In the very dilute limit (< 0.1at.%) each domain behaves as a non-interacting dipolar entity with a single relaxation time. At higher concentrations of disorder, however, the domains can interact leading to more complex relaxational behaviour. Among the manifestations of such behaviour is the formation of a glass-like relaxor (R) state, or even an ordered FE state for a sufficiently high concentration of overlapping domains. After a brief discussion of the physics of random-site electric dipoles in dielectrics, this review begins with the simplest cases, namely the relaxational properties of substitutional impurities (e.g., Mn, Fe and Ca) in the quantum paraelectrics KTaO₃ and SrTiO₃. This is followed by discussions of the relaxational properties of Li-and Nb-doped KTaO₃ and of the strong relaxors in the PbMg_1/3Nb_2/3O₃ and La-substituted PbZr_1-xTi_xO₃ families. Some emphasis will be on the roles of pressure and applied dc biasing electric fields in understanding the physics of these materials including the R-to-FE crossover. (topical review)