This synthesis on underwater bridge maintenance and repair will be of special interest to bridge maintenance engineers and others involved with repair of bridge elements located below the waterline and in the splash zone. Procedures for correctly assessing the problem and engineering the repair within the constraints of individual situations, including bridge scour considerations, are described. Protection, maintenance, and repair of concrete, masonry, steel, and timber bridge foundation elements are also addressed. This synthesis updates the maintenance and repair portion of NCHRP Synthesis 88: Underwater Inspection and Repair of Bridge Substructures. This report of the Transportation Research Board identifies and describes current practices and innovative repair techniques for problems associated with settlement, scour, and deterioration of underwater bridge elements. The synthesis considers various types of protection, maintenance, and repair work on bridge substructures, both underwater and in the splash zone, and includes techniques, and their effectiveness. Current research in the topic area and critical research needs are also identified. A limited bibliography is also provided.  The report for this topic can be purchased at https://www.trb.org/news/blurb_detail.asp?id=3357