Over the last one decade or so, a large number of 2D layered materials have been discovered. Owing to the surface-only character of the 2D crystals, the immediate environment (substrates and/or ambient gases) can alter their properties in unpredictable ways. However, most first-principles calculations are performed on free-standing 2D-crystals in vacuum. This is done for the sake of expedience, and to avoid prohibitively high computational costs. On the other hand, in the real-world, experiments are performed and devices are made with 2D crystals that are a part of composite structures. These composite structures may include stacks of 2D crystals or conventional substrates. In addition, these 2D materials may also be exposed to the ambient gases. In this talk, I will discuss how interfacial effects can critically influence different properties of 2D materials. These results show that the external influences should be taken into account for accurate predictions of the properties of the 2D crystals, which are more often than not part of composite systems.