Fibre Reinforced Polymer bridge decks, thanks to their beneficial properties and various advantages over traditional materials, have great potential as a material used in bridge engineering. They exercise high specific strength and stiffness-to-weight-ratio, a property particularly interesting from the point of view of designers, as it provides the possibility to consider new design concepts and what´s more significant, enables dead load savings, which is particularly important while retrofitting existing structures by replacing old bridge decks. Over the last 30 years, many authors have studied and conducted research for FRP composite bridge deck which are gaining greater acceptance as the materials of choices for civil and infrastructure applications. Nevertheless, many aspects still need to be addressed before a widespread introduction of this new technology is possible. The purpose of this research is to provide an overview and investigate the performance of FRP decking system used to upgrade old movable bridges when composite action is provided between the deck adhesively bonded with the main girders. This thesis will firstly introduce the literature research investigating the structural analysis of FRP decking systems. Afterwards, the efficiency of using FRP decks for upgrading old movable bridges is investigated through three-dimensional finite element models based on the use of commercial software Abaqus. Three different FRP decks, Ecosafe from Lightweight Structures BV, ASSET from Fiberline and Duraspan from Martin Marietta Composites are used to investigate the upgrading and deck replacement of the old movable bridge, Wilhelminabrug located in Zaandam, The Netherlands. Due to the deterioration of the steel superstructure, a preliminary investigation for each deck is computed using three different configuration of steel superstructure, alternating the main girder span and the availability of cross beams, during the static analysis in order to find the most suitable solution. The composite behaviour of the bridge and lateral load distribution are further examined for each of three decks using the strain distribution. Moreover, the fatigue life assessment is analyzed using the cumulative damage method for the FRP deck and the simplified ?-method for the steel superstructure. Lastly, the effect of temperature differences in vertical deflection is considered and a small parametric study is considered. Based on the results of this research, it can be concluded that FRP decking systems offer remarkable potential when low self-weight is a crucial demand of the project. The investigation suggests that the deflection (SLS) governs the design as opposed to strength (ULS), although with the right superstructure configuration the requirements can be met. Composite action and lateral load distribution are additional advantages that adhesively bonded FRP decks offer in bearing the external loadings. Upgrading old movable bridges with FRP decks acting compositely with steel girders is to be considered as a feasible option their rehabilitation.