The effects of physical adhesion on the mechanical properties of a composite structure are examined in this work. A criterion for optimum adhesion between matrix and reinforcing fibres is proposed based on maximizing the wetting tension. It is shown that the maximum wetting tension criterion best fulfils two im portant requirements for a strong interface:(i) the physical interactions at the molecular level between the resin and the fibres must be maximized, and (ii) the liquid resin must spontaneously wet the fibre surface in order to minimize the flow density at the interface. The conditions on the surface energy of the various phases leading to maximum wetting tension are analysed considering three mixing rules : two based on dispersive-polar interactions, and a third one based on acid-base interactions. The optimum adherend for a given adhesive, and the optimum adhesive for a given adherend, are examined. The analysis shows that maximum wetting tension is obtained when the substrate a nd adhesive surface energies are very high a nd equal, so that their polar and dispersive components are equal when the polar-dispersive mixing rule is used, and e.g. their Lifshitz-van der Waals’ components are equal and the acid component of one phase is equal to the basic component of the other phase when the acid-base approach is considered. It is shown using data from the literature that interfacial strength correlates with the wetting tension for fibre reinforced composites. Additional observations show that under poor wetting conditions the voids tend to concentrate at the fibre-resin interface, whereas under favourable wetting conditions they tend to coalesce in regions away from the fibre surface.