  Fabricating van der Waals (vdW) bilayer heterostructures (BL-HS) by stacking
the same or different two-dimensional (2D) layers, offers a unique physical
system with rich electronic and optical properties. Twist-angle between
component layers has emerged as a remarkable parameter that can control the
period of lateral confinement, and nature of the exciton (Coulomb bound
electron-hole pair) in reciprocal space thus creating exotic physical states
including moir\'e excitons. In this review article, we focus on opto-electronic
properties of excitons in transition metal dichalcogenide (TMD) semiconductor
twisted BL-HS. We look at existing evidence of moir\'e excitons in localized
and strongly correlated states, and at nanoscale mapping of moir\'e
superlattice and lattice-reconstruction. This review will be helpful in guiding
the community as well as motivating work in areas such as near-field optical
measurements and controlling the creation of novel physical states.
