This study investigated the effect of fibre content, fibre treatment and fibre/matrix interfacial strength on the mechanical properties of industrial hemp fibre reinforced polylactide (PLA) and unsaturated polyester (UPE) composites. Surface treatment of hemp fibres was investigated as a means of improving the fibre/matrix interfacial strength and mechanical properties of hemp fibre reinforced PLA and UPE composites. The fibres were treated with sodium hydroxide, acetic anhydride, maleic anhydride and silane. A combined treatment, sodium hydroxide and silane, was also carried out. The average tensile strength of sodium hydroxide treated fibres (ALK) slightly increased compared with that of untreated fibres, which was believed to be as a result of increased cellulose crystallinity. In contrast, the average tensile strength of acetic anhydride, maleic anhydride and silane treated fibres slightly decreased compared with that of untreated fibres, which was believed to be as a result of decreased cellulose crystallinity. In the case of combined treatment with sodium hydroxide and silane, the average tensile strength of the fibres (ALKSIL) slightly decreased compared with that of alkali treated fibres (ALK), which was also thought to be as a result of decreased cellulose crystallinity. The average Young's modulus and thermal stability of all treated fibres increased compared with untreated fibres. This was considered to be as a result of densification of fibre cell walls due to the removal of non-cellulosic components during treatment. It was also thought that the grafted molecules in cellulose chains of the acetic anhydride, maleic anhydride and silane treated fibres enhanced resistance to thermal degradation. The interfacial shear strength (IFSS) of PLA/hemp fibre samples increased after treatment, except in the case of maleic anhydride treatment. The increase in IFSS could be due to better bonding of PLA with cellulose of treated fibres (except for the maleic anhydride treatment) as a result of removal of non-cellulosic components evidenced by increased PLA transcrystallinity. The highest IFSS was 11.41 MPa which was obtained for PLA/ALK samples. IFSS of UPE/hemp fibre samples increased for all treated fibres. This could be due to the improvement of chemical bonding between the treated fibres and the UPE as supported by FT-IR results. The highest IFSS (20.3 MPa) was found for the UPE/ALKSIL samples. Short hemp fibre reinforced PLA composites were fabricated using injection moulding. Alkali and silane fibre treatments were found to improve mechanical (tensile, flexural and impact) and dynamic mechanical (storage modulus) properties which appears to be due to the increase in IFSS and matrix crystallinity. Tensile strength, Young's modulus, flexural modulus, impact strength and storage modulus of the PLA/hemp fibre composites increased with increased fibre content. A 30 wt% short fibre reinforced PLA composite (PLA/ALK) with a tensile strength of 75.5 MPa, Young's modulus of 8.18 GPa, flexural modulus of 6.33 GPa, impact strength of 2.64 kJ/m² (notched) and 28.1 kJ/m² (un-notched) and storage modulus of 4.28 GPa was found to be the best, and better than any in the available literature. However, flexural strength, plane strain fracture toughness (Kic) and strain energy release rate (Gic) decreased with increased fibre content. This behaviour could be due to the increase in stress concentration points (number of fibre ends) with increased fibre content. The influence of loading rate and fibre content on the Kic of random short fibre reinforced PLA composites, lacking from the available literature, was studied. Kq (trial Kic) of composites decreased as loading rate increased, until stabilising at a loading rate of 10 mm/min and higher. UPE based short hemp fibre composites were produced by compression moulding. At 20 wt% fibre content, the tensile strength was not increased above that recorded for unreinforced UPE, but thereafter, the tensile strength increased approximately proportionally to the fibre content, except at the highest fibre content (60 wt%) where a decrease in the tensile strength occurred. Kic and Gic reached a minimum value at 30 wt% fibre content and afterwards increased with increased fibre content. The flexural strength was found to decrease with increased fibre content; however, the impact strength and storage modulus increased with increased fibre content. It was also observed that the mechanical and dynamic mechanical properties improved after treatment of fibres. A 50 wt% ALKSIL fibre reinforced UPE composite with a tensile strength of 62.1 MPa, Young's modulus of 13.35 GPa, flexural modulus of 6.11 GPa, impact strength (notched) of 7.12 kJ/m² and storage modulus of 3.5 GPa was found to be the best. To improve the mechanical and dynamic mechanical properties further, aligned long hemp fibres were used to fabricate PLA/ALK and UPE/ALKSIL composites using compression moulding. The mechanical and dynamic mechanical properties of aligned long fibre reinforced PLA/ALK and UPE/ALKSIL composites were found to be superior to those of short fibre composites. The highest tensile strength of 85.4 MPa, Young's modulus of 12.6 GPa, flexural modulus of 6.59 GPa, impact strength of 7.4 kJ/m² (notched) and 32.8 kJ/m² (un-notched), and storage modulus of 5.59 GPa were found for PLA/ALK composites at a fibre content of 35 wt%. In the case of UPE/ALKSIL composites, the highest tensile strength of 83 MPa, Young's modulus of 14.4 GPa, flexural modulus of 6.7 GPa, impact strength (notched) of 15.85 kJ/m² and storage modulus of 3.74 GPa were found for composites with a fibre content of 50 wt%.