This synthesis will be of interest to state department of transportation bridge design and structural engineers, bridge consultants, and others involved in applied and research methods for increasing the live load capacity of existing highway bridges. The synthesis describes the current state of the practice for the various methods used to increase the live load capacity of existing highway bridges. This is done predominantly for bridges in the short- to medium-span range. Information on the more common bridge material types is presented. There is an emphasis on superstructure rather than substructure strengthening.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3308