This study deals with medium density wood fibre (MDF) and Kraft fibre reinforced polypropylene (PP) composites produced using extrusion followed by injection moulding. Initially, composites were produced with MDF fibre using 10, 20, 30, 40, 50 and 60 wt% fibre, and 1, 2, 3 and 4 wt% maleated polypropylene (MAPP) as a coupling agent. A fibre content of 50 wt% with 3 wt% MAPP was found to be optimum. Alkali treatment of fibre was carried out to improve the interfacial bonding. After treatment, fibre surface charge was found to increase, but single fibre tensile strength (TS) and Young's modulus were (YM) decreased. Alkali treatment reduced composite TS but increased YM. The effects of hemicellulose and residual lignin content were assessed with Kraft fibre (subjected to different stages of a standard Kraft pulping process and therefore consisting of different hemicellulose and residual lignin contents). Fibre surface charge was found to increase with decreasing residual lignin content. Composites containing higher amounts of lignin lead to lower TS and lower thermal stability. Composites were subjected to accelerated weathering for 1000 hours. TS and YM were found to decrease during weathering, and the extent of reduction was found to be higher for composites with higher residual lignin. The reduction of mechanical properties was found to be due to degradation of lignin and PP chain scission as evaluated by increase in PP crystallinity after weathering. As low lignin (bleached) Kraft fibre composites were found to provide superior mechanical properties, as well as more stable during accelerated weathering, further study including optimisation of MAPP content, effects of fibre contents, fibre length, fibre beating, hygrothermal ageing and recycling were carried out with bleached Kraft fibre. MAPP contents of 1, 2, 3, 4, 5, 7 and 10 wt% were used in Kraft fibre reinforced PP composites, and 3-5 wt% was found to be most favourable. Composite fibre content was varied between 30-50 wt%, and 40 wt% found to provide the maximum TS. To investigate the effects of fibre length on composites, fibre fractions of different length distribution were separated using a pressure screen. TS, YM and impact strength were found to decrease and failure strain (FS) increased with decreasing fibre length. To improve the interfacial bonding, the fibre was treated by mechanical beater. Fibre beating increased the TS of composites up to a certain point, beyond which TS decreased. Hygothermal ageing of composites was carried out by immersing specimens in distilled water at 30, 50 and 70 C over an 8-month period. Equilibrium moisture content and diffusion coefficient increased with increased fibre content in composites as well as with increased immersion temperature. Composites without coupling agent showed higher water uptake and diffusion coefficient than that of with coupling agent. After hygrothermal ageing the TS and YM decreased but FS and impact strength were found to increase. An investigation into the effects of recycling was carried out with composites containing either 40 wt% or 50 wt% fibre (bleached Kraft) with 4 wt% MAPP, and recycled up to eight times. For composites with 40 wt% fibre, TS and YM were found to decrease with increased recycling by up to 25% for TS and 17% for YM (after being recycled 8 times). Although TS was lower for virgin composites with 50 wt% fibre than for those with 40 wt% fibre, this initially increased with recycling by up to 14% (after being recycled 2 times), which was considered to be due to improved fibre dispersion, but then decreased with further recycling, and an overall 11% reduction of TS was found after recycling 8 times compared to the virgin composites. YM was higher for virgin composites with 50 wt% fibre than those with for 40 wt% fibre, and also initially increased with recycling but decreased upon further recycling. Recycling was found to increase thermal stability. The TS of composites made by combining recycled with virgin materials was also assessed. Hygrothermal ageing behaviour of recycled composites was also investigated by immersing specimens in distilled water at 50 C over a 9 month period. It was found that the diffusion coefficient and the equilibrium moisture contents of composites decreased with increased number of times the materials were recycled. After hygrothermal ageing, TS and YM of composites were found to decrease. However, the extent of reduction was found to decrease with increased recycling.