This paper deals with the influence of interphase formation on the stress transfer capacity of the fibre-matrix interface in single fibre composites and constitutes a brief review of our recent work. This influence is studied through the deviation of the experimental results from a recent theoretical analysis relating the interfacial shear strength to the reversible work of adhesion established between the fibre and the matrix. Two examples of interphases are considered: a transcrystalline layer in carbon fibre-PEEK composites and a pseudo-glassy interphase in carbon fibre-elastomer systems. In the former case, the low transverse mechanical properties of the transcrystalline layer lead to a decrease of the interfacial shear strength, whereas, in the latter case, the glassy behaviour of the interphase explains the improved interfacial stress transfer capacity.