In this thesis we have investigated electrostatic interactions at dielectric interfaces using theoretical models based on the non-linear Poisson-Boltzmann theory and its extensions. We have focused on three major topics: (1) modeling the energetics and interactions of charged nanoparticles trapped at the air-water interface; (2) calculation of the line tension between domains in charged lipid membranes, lipid-lipid correlations, and how membrane curvature is influenced by charged peptides; and (3) extensions of the classical Poisson-Boltzmann theory by accounting for the influence of ion-specific solvent-mediated interactions. More precisely, ion-specificity has been accounted for using the Poisson-Helmholtz-Boltzmann formalism, which adds to the bare Coulombic interactions a Yukawa-like potential that accounts for the interacting hydration shells of ions. Motivated by recent experimental and computational results, all projects present here aim to provide a deeper understanding of fundamental physical properties of charged dielectric interfaces.