Two-dimensional materials, such as graphene, exhibit various unique electronic and optical properties that distinguish them from their bulk parent compounds. Besides being highly tunable by electrostatic gating, these 2D materials can be assembled into van der Waals heterostructures, which greatly extend the possibilities one can achieve. Among these possibilities, the twist angle in a van der Waals heterostructure is a unique knob, which we can utilize to engineer the properties of the 2D materials in unprecedented ways. In this colloquium, I will mainly talk about the electronic properties of twisted bilayer graphene (TBG), consisting of two pieces of graphene rotated by a certain angle. It is shown experimentally that the twist angle significantly alters the band structure, by reducing the Fermi velocity at the Dirac points and by inducing new band gaps, due to the formation of a moiré superlattice. In particular, at a ‘magic’ twist angle, the band structure becomes strongly flattened, to an extent that the Coulomb interactions between the electrons now become dominant. In such a regime, peculiar correlated insulator states and unconventional superconductivity are found, which share some traits with those observed in high-Tc superconducting materials. These findings established the first graphene superconductor in two dimensions. I will also talk about extensions on TBG that lead to a family of moiré superconductors which share a lot of similarities, while also having important differences. These studies might help us understand more about the physics in flat-band systems, which might in turn shed more light towards the underlying mechanisms in other strongly correlated materials and unconventional superconductors.