A combined experimental and numerical study has been carried out in order to predict initial failure in transversely loaded carbon fibre/epoxy composites. Three-point bending experiments on macroscopic composite specimens with special laminate lay-ups were carried out in a scanning electron microscope (SEM). The in-situ experiments allow observing the onset of microscopic composite failure under transverse loading and measurement of the global applied load at onset of failure. The experimental results show that interfacial failure was the dominating failure mechanism. The interfacial stresses at initiation of failure were determined successfully by a non-linear micro/macro FE-Analysis and compared with experimental results obtained from 3-point bending tests of standard composite specimens. The results show that the interfacial normal strength (INS) governs the failure process.