This report details the results of a study of the various methods of strengthening highway bridges. The initial task in this study was a thorough review of pertinent international literature to determine the methods currently being used and to discover innovative ideas being considered. An extensive overview of all methods is presented. The types of structures most suitable for strengthening are identified, and the cost-effectiveness of the various methods is presented for these structures. New materials and innovative techniques are presented. The major effort of the study was developing a strengthening manual for use by practicing engineers. This manual describes the most effective techniques and indicates how they may be used in various types of structures.