This thesis explores the use of lightweight composite materials in road freight trailer design as a means of reducing the emissions of the road freight industry. A comprehensive review of previous lightweight composite trailers and related projects was conducted; it concluded that the application of composites in trailers to-date has largely been limited by relatively high material and production costs. The review highlighted that the trailer industry could learn from the success of composites in the bridge construction industry. A statistical weight analysis of two road freight fleets and an energy consumption estimation, via a drive cycle analysis, were used to identify trailers that are particularly suited to lightweighting. Hardwood trailer decking was identified as a prime subcomponent for composite replacement. However, there is little literature on how conventional hardwood trailer decks react to in-service loadings. This problem was addressed through a comprehensive deck damage study, which was used to benchmark novel lightweight deck systems. Several lightweight replacement composite sandwich panels were designed, built and tested. Two different pultruded GFRP decks were also examined. While pultrusions do not offer the same level of weight savings as sandwich panels, the highly cost-driven nature of the trailer industry could dictate that their integration is the most reasonable first step to introducing composites into structural subcomponents. The final part of the thesis explores options for lightweighting the trailer chassis holistically. Trailer load cases were investigated through finite element modelling in Abaqus. A parametric model of a typical longitudinal trailer I-beam was developed using Python scripting and Abaqus. The model was expanded to analyse composite trailer structures. It showed that approximately 1,300 kg of weight could be saved by shape and material optimisation in a composite trailer. In summary, this research has shown that short-term trailer weight reductions can be effectively achieved through subcomponent replacement, while more significant reductions can be achieved in the long-term by a ‘clean slate’ composite redesign of the trailer chassis. The lightweighting strategies presented here are poised to have an increasingly important role in reducing the emissions of the road freight industry.