Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. From a material physicist’s point of view, 2D materials provides vast opportunities on two fronts: first, the reduced dimensionality in these 2D crystals often leads to novel material properties that are different from those in the bulk; second, the entire 2D crystal is a surface, so it is possible to have better control of their material properties with external perturbations. In this talk I will illustrate these two points with a few examples. These layered materials range from semiconductors and high temperature superconductors. In particular, black phosphorus is a 2D semiconductor, and its superior material quality has recently enabled us to observe the quantum Hall effect; others include magnetic topological insulator and high temperature superconductor Bi2Sr2CaCu2O8+ in the 2D limit. We explore their electronic properties while the doping and dimensionality of the 2D systems are modulated.