A thorough investigation of the failure mechanisms of composite sandwich beams under four- and three-point bending and cantilever beams was undertaken. The beams were made of unidirectional carbon/epoxy (AS4/3501-6) facings and a PVC closed-cell foam (Divinycell) core. Two types of core material H100 and H250 with densities 100 and 250 kg/m3, respectively, were used. The failure modes investigated are face sheet compressive failure, core failure and facing wrinkling. The various modes have been studied separately and both initiation and ultimate failure have been determined. Initiation of a particular failure mode and triggering and interaction with other failure modes was also investigated.