Abstract: In a flat band system, the charge carriers’ energy-momentum relation is very weakly dispersive. The resultant large density of states and the dominance of Coulomb potential energy relative to the kinetic energy favor the formation of strongly correlated electron states, such as ferromagnetism, nematicity, and superconductivity. The advent of two-dimensional (2D) materials and their heterostructures has ushered in a new era for exploring, tuning and engineering of flat band system. Here I will present our recent works in twisted bilayer graphene, including experimental demonstration of the resolution of the paradox of slow Fermi velocity by quantum geometry, and suppression of superconductivity by in situ tuning of Coulomb screening.