Composite primary and secondary aircraft structures have the potential to achieve the same strength and stiffness as conventional metallic structures, but at substantially lower weight. To accomplish this in an efficient and effective manner the joints used to attach these composite structures will be critical. This work investigated the strain distribution in adhesively bonded and hybrid bolted/bonded composite single-strap butt joints using a 2D digital image correlation (DIC) technique and geometrically nonlinear finite element modelling. A high magnification optical setup was used to measure the strains in the through thickness direction, allowing strain measurements in the region of the adhesive and along the bondline in the joint overlap section. This provided a detailed measure of strain flow through the adhesive and into the doubler and adherend under a maximum tensile load of 5338 N (1200 lbf). In all cases a crack developed at the inner overlap section in the hybrid joints and, using DIC techniques, the size of the crack as well as crack tip strains could be determined from the deformed images. The information obtained from the DIC measurements proved valuable in helping to improve the fidelity of the finite element model.