The PhD research is focused on the reinforcement of fatigue cracked orthotropic steel bridge decks (OBD) by adding a second steel plate to the existing deck. The main idea is to stiffen the existing deck plate, which will reduce the stresses at the fatigue sensitive details and extend the fatigue life of the orthotropic bridge decks. Two reinforcement systems are subject of research, the bonded steel plates system and the sandwich steel plates system. In the bonded steel plates system, the existing OBD is reinforced by adding the second steel plate, using a thin epoxy adhesive layer (approx. 2 mm thick), vacuum infused between the two steel plates. In the sandwich steel plates system, the existing deck is reinforced by adding a sandwich overlay which is composed of a polyurethane core (PU-core, from 15 mm to 30 mm thick) and the second steel plate. The performance of the reinforcement systems was evaluated through extensive experimental and numerical studies. The behaviour of both reinforcements was tested under static and fatigue loading, including full-scale tests on reinforced deck-panels and structural monitoring of a real-scale application. The results show a significant stress reduction at the fatigue sensitive details after applying the reinforcement systems. Furthermore, realistic wheel loads did not cause fatigue damage in any of the reinforcement systems. Overall, the performance of these two light-weight solutions for reinforcing OBD proved to be efficient and durable. A step forward in the design approach to evaluate the reinforcement of OBD has been established.