The effect of pressure -by definition a 3-dimensional concept -on 2-dimensional systems brings a number of fundamental questions, which have been partly answered through newly engineered samples and experimental setups. In this review of the high-pressure Raman studies of graphene, we will in particular underline the importance of the presence of a supporting substrate and its role for the production of biaxial strain conditions in high pressure experiments. Raman shifts observed during these experiments may be related to the strain induced by the substrate rather than by the applied pressure, graphene stress, and hydrostatic pressure not being straightforwardly equivalent in this case of a composite 2D-on-3D material. We will also consider the effects of the nature of the substrate surface, the nature of the pressure transmitting media, as well as the effect of the number of layers on the high-pressure response of the 2D-system, by using the phonon behavior as strain probe as pressure is applied. The effects of adhesion and of strain transfer from the substrate to the 2D-system will be at the heart of our discussion.

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