One of the greatest limitations to the use of polymer composites is the high cost of the techniques traditionally employed in their manufacture. Therefore, less Labour intensive, lower cost manufacturing methods such as resin transfer moulding (RTM) have been developed. Such processes require a resin with low viscosity at the processing temperature. Resins that exhibit good high temperature performance tend to be viscous, and are usually brittle when cured. Conventional toughening agents, such as polyethersulfone, further increase the viscosity of the blend. This work focuses on the use of hyperbranched polymers as resin additives, as these low viscosity materials can have the potential to act as toughening agents. It was hypothesised that the thermal properties of the blend could be simultaneously enhanced by the incorporation of a thermally stable inorganic phase, and as such the synthesis of organic-inorganic hybrids was investigated. Organic-inorganic hybrids of a commercially available hyperbranched polyester, Boltorn E1, were synthesised using two hydrolytic sol-gel methods. In both methods the inorganic phase was silica; one process was carried out in solution, the other was an emulsion method. The sol-gel processes were carried out under comparatively mild conditions and so allowed the introduction of organic material that would be degraded by high temperature processing. Both methods yielded nano-scale hybrid products, as confirmed by electron microscopy and infrared spectroscopy studies. Particulate hybrids of Boltorn E1 and silica, of approximately 100 nm diameter, were successfully synthesised using the emulsion method. The use of hyperbranched polymers as additives to epoxy resins was investigated, both as the organic component of organic-inorganic hybrid additive, and as resin modifiers in their own right. The addition of Boltorn E1 failed to increase the fracture toughness of a variety of epoxy resins. The reasons for this were identified. The addition of hybrid particles did not enhance the thermal properties of a trifunctional epoxy resin, as the additive sterically inhibited resin cross-linking. It was hypothesised that reducing particle size and agglomeration would prevent this plasticising effect.