Most fascinating phenomena are observed in materials where various interactions happen on the same energy scale. Among them are organic-inorganic hybrid systems or interfaces between 2D materials. Another example is provided by combinations of polar and nonpolar building blocks where 2D electron gases (2DEG) form at their interface. For an ab initio description of their electronic excitations, all these materials are critical examples, being governed by the interplay between electron-electron interaction, electron-phonon coupling, electron-hole correlations, and potentially also spin-orbit coupling (SOC). A particular role is also played by the mutual dynamical screening of the interface constituents. With selected examples I will show how hybrid and charge transfer excitations can form at such interfaces, or how a 2DEG can be obtained and then manipulated by an electric field. I will also discuss the importance of forefront many-body methodology since density-functional theory (DFT) may well fail in predicting the level alignment and consequently optical and transport properties.