The use of natural fibres as replacement for traditional glass fibers in composites has gained increasing importance in recent years as environmental concerns have led to a quest for sustainable alternatives. However natural fibre composites have generally been found to have limitations in their mechanical properties. This book presents the results of studies undertaken to explore the mechanical properties, especially impact and fatigue, of hemp fibre reinforced polyester composites. The specific tensile properties and impact damage tolerance of hemp fibre composites were found to be low compared to glass fibre composites. Despite having poorer absolute fatigue strength, hemp fibre composites exhibited better fatigue sensitivity than glass fibre composites in tension-tension fatigue. The effects of water immersion and various fibre surface treatments on the impact and fatigue properties of hemp fibre composites were also studied.