The development of theories for the electrohydrodynamics at soft surfaces enabled major progress in the quantitative interpretation of streaming current and surface conductivity data collected for polymer brushes. In this paper, we review the basics of the methodology and discuss illustrative examples of practical interest. In particular, we demonstrate how the combination of self-consistent field and soft surface electrokinetic theories using Poisson–Boltzmann (PB) formulations allow the evaluation of the segment density distribution within poly(ethylene oxide) brushes beyond the resolution limits of neutron reflectivity. The application of the methodology for the analysis of the charge, structure, and pairing with chaotropic anions is illustrated for strong cationic poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) brushes. Finally, we report refinements of the PB theory that account for ion hydration, ion pairing, and dielectric decrement in brushes and we present an example of glycosaminoglycan brushes where those effects are significant