This thesis is divided in four parts, namely: a local strength approach, a local fracture-based approach, the assessment of the strength of structural joints and the assessment of defect tolerance criterion. The local strength of approach deals with the assessment of an adhesive system (adhesive - primer - surface preparation) and selected adherends for different ageing conditions. At this stage, the particular focus is to characterise the change of behaviour due to increasing adhesive bond thickness through both experimental and finite element models. It is outlined that the shear strength of the joints decreases in a linear manner with the adhesive bondline thickness and a failure mechanism of the joint is proposed thanks to experimental results and finite element analysis. The local fracture-based approach deals with the assessment of fracture toughness of Double Cantilever Beam (DCB) adhesive joints. This analysis is carried out using analytical and numerical models which incorporate experimental data from the DCB tests. The adhesive system exhibits a high fracture toughness and the study underlines the influence of the bondline thickness on the fracture toughness of the joint while it is shown that accelerated ageing process lowers it. The structural joint assessment focuses on the joint behaviour in a tensile mode and aims to explain the mode of failure of two different types of joints of different adherend materials and different adherend thicknesses. The experiments and the numerical analysis helped to locate stress concentration areas and showed that failure occurs mainly due to tensile stress.