The stress transfer between a single fibre and a matrix has been studied using the technique of automated phase-stepping photoelasticity. The contours of isochromatic fringe order provide the locations and magnitude of maximum shear stress. A model composite in which a fibre-break leads to the propagation of a matrix crack has been used to compare the interfacial shear stresses around bonded and debonded interfaces in the absence and presence of a fibre-break and matrix cracks.