Experiments have been carried out on the propagation of cracks in resin beams containing regularly spaced glass fibres. The effects of the fibre spacing, orientation and surface treatment on the fracture energy and cracking resistance of these composites are discussed with reference to acoustic emission records and polarized light observations of the fracture process. Analysis of curves of load against deflexion shows that the major factor contributing to the fracture energy of the composite is work done against friction between fibre and matrix. It has been suggested that the fracture energy and stress intensity approaches to understanding fracture-resistance of composites should give the same result, and indeed some of the results of this work appear to support this view. But a detailed examination of the relative effects on cracking of the fibre spacing and fibre volume fraction - ostensibly the same thing - suggests that the fracture energy is determined by factors somewhat different from those which govern the stress intensity factor. The fracture energy is controlled simply by the fibre content, irrespective of their spacing, whereas the critical stress intensity factor is affected separately by fibre spacing and volume fraction.