Increasing worldwide environmental awareness is encouraging scientific research into the development of cheaper, more environmentally friendly and more sustainable construction and packaging materials. Natural fibre reinforced thermoplastic composites are strong, stiff, lightweight and recyclable, and have the potential to meet this need. Industrial hemp fibre is amongst the strongest of the natural fibres, and possesses a similar specific strength and stiffness to Eglass, but with additional benefits such as low cost and low production energy requirements. The favourable mechanical properties of hemp, however, have yet to be transferred successfully to thermoplastic-matrix based composite materials. The main objective of this research was to produce an improved hemp-fibre reinforced polypropylene composite material by optimising the fibre strength, fibreprocessing methods, composite-processing methods and fibre-matrix interfacial bonding. To obtain the strongest and stiffest fibres for use in composites, an investigation was performed on a crop of New Zealand grown hemp to determine the effects of plant growth duration on fibre strength and stiffness. By conducting single fibre tensile tests on retted hemp fibre, it was discovered that the optimum cultivation time was 114 days, producing fibres with an average tensile strength of 671 MPa, and a Young's modulus of 40 GPa. It was also found that the retting process considerably reduced the tensile strength and stiffness of the hemp fibres. Biological fibre-treatments using 3 varieties of fungi were then used to modify the surface morphology of hemp fibres, so that the adhesion between the fibres and a polypropylene matrix could be improved. It was found, however, that these treatments severely degraded the fibres, and reduced their tensile properties to such an extent that they were deemed unsuitable for use as composite reinforcements. An investigation was then conducted to determine a suitable fibre treatment method to remove lignin and other non-structural constituents from the fibre wall, to separate the fibres from their fibre bundles, and to retain the fibre strength. Various alkali treatments were used, and a treatment consisting of fibre digestion in a 10% NaOH solution with a maximum processing temperature of 160°C and a hold time of 45 minutes, was found to provide the best combination of fibre strength retention, lignin removal and fibre separation. Finally, the alkali treated fibres, polypropylene and a maleated polypropylene (MAPP) coupling agent were compounded using a twin-screw extruder, before being injection moulded into composite tensile test specimens. A range of composites with differing fibre and MAPP weight contents were then produced and tensile tested. It was found that the addition of 2% MAPP greatly improved the tensile strength and interfacial bonding of the 30% fibre composites, while only moderate improvements were observed for the 40% fibre composites. An increase in the fibre content from 0% to 40% greatly improved the stiffness of the composites. The most successful composite produced consisted of 40% long alkali treated fibres with 2% MAPP, and had a tensile strength and Young's modulus of 39 MPa and 4.4 GPa respectively.