Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers, a class of ultrathin materials with a direct bandgap and
high exciton binding energies, provide an ideal platform to study the photoluminescence (PL) of light-emitting devices. Atomically
thin TMDCs usually contain various defects, which enrich the lattice structure and give rise to many intriguing properties. As the
influences of defects can be either detrimental or beneficial, a comprehensive understanding of the internal mechanisms underlying
defect behaviour is required for PL tailoring. Herein, recent advances in the defect influences on PL emission are summarized and
discussed. Fundamental mechanisms are the focus of this review, such as radiative/nonradiative recombination kinetics and band
structure modification. Both challenges and opportunities are present in the field of defect manipulation, and the exploration of
mechanisms is expected to facilitate the applications of 2D TMDCs in the future.