Carbon fibre composites, where bundles of carbon fibres are embedded in a polymer matrix, offer enormous design flexibility due to the ability to tailor properties such as strength and modulus. They are used to lightweight aerospace and automotive structures and are seeing an exponential increase in demand with the global market predicted to grow to $36 billion by the year 2020. The Achilles heel of these materials lies in their ability to withstand damage from impact loads. Once damaged, the ultimate strength of the material is reduced via the formation of microcracks that lead to delamination failure. The microcracks are often found to originate at the interface between the fibre and the matrix, yet interfacial adhesion in composite materials is poorly understood and still hotly debated in the literature. The relative influence of fibre grade (standard, intermediate or high modulus), fibre surface roughness and chemistry in interfacial adhesion and therefore microcracking is required. This new knowledge will lead to the development of engineered interfaces in composites for improved performance. There are many examples of tailored interfaces in nature from which we can draw inspiration. Nacre (mother of pearl) is a natural high-performance composite comprising nanocrystals of CaCO3, aragonite, embedded in a multi-layered, multi-component soft organic matrix. The CaCO3 crystals do not bind directly to the organic chitin layer, but are separated by an intermediate protein layer. This intermediate phase creates a soft/soft/hard interface that can be likened to an engineered interphase in carbon fibre composites. Commercial carbon fibres are usually electrolytically oxidised and then coated with either an epoxy or a polyurethane sizing to protect the fibre during processing and to facilitate handling. The nature and presence of this sizing may influence the chemistry of the surrounding polymer matrix to create an interphase region. We have explored a range of strategies to tailor the fibre surface roughness and chemistry to ensure compatibility with the polymer matrix. The electrolytic treatment was systematically varied to ensure that the fibre surface was free of amorphous carbon debris, a range of chemical treatments were used including the attachment of small molecules to the fibre surfaces. Novel sizing systems and plasma polymer coatings have also been applied to the fibres. This paper will describe our recent results on the effect of a range of fibre treatments to improve interfacial performance in carbon fibre composites.