The principle objective of this project was to optimise extraction of cellulose nanofibres from banana tree and rye grass feedstocks and to investigate the potential use of these products as high value-reinforcing agents in thermoplastic polymers, particularly in polyvinyl alcohol (PV A) and polyethylene (PE). To this end, in association with the Agri-Food and Biosciences Institute (AFBr) in Belfast, the following extraction techniques were investigated to obtain cellulose nanofibres from these natural fibre sources: (i) chemical modification, involving mercerisation, acid hydrolysis and chemical bleaching; (ii) mechanical treatment, using a high speed (Turrax) mixer and high pressure microfluidisation and (iii) chemical and mechanical (chemo-mechanical) processes, by combining TEMPO-oxidation and high pressure microfluidisation. The nanofibres produced were characterised using scanning electron and transmission electron microscopy, particle size measurement (static image analysis and laser diffraction), chemical analysis (zeta potential analysis, fourier transform infrared spectroscopy and x-ray diffraction), and thermogravimetric analysis. It was especially evident that the chemo-mechanical procedure yielded higher aspect ratio nanofibrils, a greater yield and higher crystallinity, than nanofibrils made by solely chemical or mechanical treatments. The cellulose nanofibres obtained were subsequently incorporated into PVA by a solution casting technique. The effect of different treated nanofibres on the mechanical, structural and thermal properties of these composites was determined. It was notable that banana nanofibres made by TEMPO-oxidation and high pressure microfluidisation showed phenomenal reinforcing effects in the PVA. Furthermore, ryegrass nanofibre, derived from the high speed Turrax mixer, was , incorporated into PV A and PE by using more commercially acceptable melt processing procedures, involving surface treatment of the nanofibres using silane coupling agents, their pre-dispersion in a PVA carrier, twin-screw extrusion compounding and compression lamination methodologies. Varying degrees of success were seen, from poor dispersion using cryogenically milled nanofibres, yielding little effect on mechanical properties, to very significant enhancement with melt processable PVA, being of a similar order to solution cast nanofibre-reinforced PVA To assess the relative reinforcing efficiency of cellulose nanofibres, conventional PE composites were also made using macro-scale banana and sisal fibres. To aid compatibility and enhance interfacial bonding between fibre and matrix in this system, maleic anhydride modified polyethylene was applied with both nano- and macro- fibre variants. By way of example, there was a 100% improvement in tensile modulus of conventional banana fibre-reinforced PE composite with a 30 wt% loading of micron-sized banana fibres, whereas a 300% improvement was recorded in tensile modulus for cellulose nanofibre-reinforced PVA with only 5 wt% of cellulose nanofibres.