Many energy-conversion processes take place at heterogeneous interfaces formed between molecules and solids at the nanoscale. Characterization of the electronic structure and interaction at adsorbate-substrate interfaces is crucial for fundamental understanding of interfacial charge dynamics. In this talk, I will introduce two concepts related to the interplay between the adsorbate and the substrate: level alignment – the energy difference between molecular frontier orbitals and the Fermi level of the substrate, and level broadening – the line-shapes of molecular resonances. In the first part, I will first discuss why common functionals of density functional theory (DFT) are less successful in predicting level alignments than calculating binding geometries and adsorption energies. After that, I will introduce new methodological advancements for accelerating first-principles calculations based on many-body perturbation theory (MBPT) that is formally rigorous in computing quasiparticle energies. We have developed two approaches, substrate screening GW and dielectric embedding GW, which have been successfully applied to a number of systems of experimental significance. In the second part, I will introduce a method to extract the phenomenological level broadening from first-principles non-equilibrium Green’s function calculations, in the context of molecular junctions. This method provides insight into exotic transport behaviors in both symmetric and asymmetric single-molecule junctions.