The research progress in the area of dynamic impact factors (IM) of highway bridges during the past two decades was reviewed in the paper. The definition of IM was first introduced. Then, related progress and findings from both field studies and numerical simulations were summarized. Parameters that affect the IMwere discussed in details and provisions relevant to the IM in bridge design codes of many countries around the world were reviewed. Finally, based on the advances achieved in this field, some conclusions were drawn and much effort was made to identify the remaining controversies and gaps left in this field.