The electronic structure of interfaces comprising electronic materials governs fundamental charge and energy transfer processes, and thus the functionality and efficiency of electronic and optoelectronic devices featuring these interfaces. Mixed cation and halide perovskites are considered prime materials for photovoltaics. Yet, the fundamental understanding of their electronic structure and the energy level alignment with charge transport layers is limited. As is discussed in this contribution, perovskite surface states and concomitant surface photovoltage effects can mask the ground state electronic properties in photoemission experiments, and only low photon dose procedures allow unraveling reliable interface energetics of relevance for devices. Two-dimensional (2D) transition metal dichalcogenides (TMD) semiconductors also emerge as highly interesting electronic materials. They feature direct energy gaps in monolayer form, and their pronounced excitonic nature offers the possibility of fine-tuning electronic and optical properties by engineering the dielectric environment. As exemplified here, the exciton binding energy of MoS2 and WSe2 can vary by a factor of two, depending on the substrate’s dielectric constant. Furthermore, charge transfer interactions with molecular electron donors and acceptors facilitate doping of TMDs. The mechanism of this type of interface doping is contrasted with that of conventional semiconductor (GaN, Si) surfaces. For the latter case, it is demonstrated how molecular donors and acceptors can be employed to tune the level alignment at inorganic/organic semiconductor heterojunctions over extreme intervals.