Dendritic hyperbranched polymers have been shown to be able to double the interlaminar fracture resistance of epoxy-based composites and to reduce the internal stress level by as much as 80% with only 10 phr of modifier. These property improvements were obtained without affecting the viscosity, and thus the processability, nor the glass transition temperature of the epoxy resin. In the investigation, both fully soluble and phase-separating epoxy-functionalised hyperbranched polymers were used, the latter showing more efficient toughening properties. In these blend formulations, however, a close control of the phase separation mechanism was required, in order to avoid filtering effects before or during fibre impregnation. In composite plaques, the phase separation was investigated as a function of fibre surface treatment. In a few cases, a heterogeneous nucleation of modifier particles occurred at the fibre surface as a consequence of favoured fibre/particle interactions. This reduced the fibre/matrix bonding strength and led to adhesive failures at the fibre/matrix interface. In using dendritic hyperbranched polymer modifiers, maximum toughness enhancement and internal stress reduction, were thus obtained when the modifier nucleated within the matrix phase and adhesive failure at the fibre/matrix interface was avoided by selecting suitable fibre surface treatments.