We review recent observations in our laboratory using atomic-resolution transmission electron microscopy (TEM), focusing on three two–dimensional (2D) materials, graphene, hexagonal boron nitride, and molybdenum tungsten disulfide – a conductor, an insulator, and a semiconductor, respectively. In all cases the atomic scale TEM observations contributed critical information to our efforts to understand the structure, defect formation, and synthesis of these fascinating 2D materials.