Results of fatigue tests on adhesive lap joints of thick (9.9mm) composite laminates are presented and discussed. Specimens of different overlap length (from 25 to 110mm), different shape, (with and without taper), and different materials (composite on composite, composite on steel) were fatigue tested and distinct fatigue curves were obtained in terms of maximum applied force and number of cycles to failure. In order to investigate the relationship between peak elastic stresses in the adhesive layer and fatigue life, a 2-D structural analysis of the joints by the finite element method was performed. The analysis evidenced a close relationship between the peak elastic stresses and the number of cycles to failure in the all-composite joints, as all experimental points in terms of peak shear (or Tresca) stress and number of cycles to failure collapsed within a band of relatively low scatter, irrespective of the shape of the joint and the overlap length. This behaviour suggested that peak elastic stresses in the adhesive layer could be adopted as a design criterion, at least as an engineering tool for industrial applications. In order for this method to be applied to the design of real components made of thick composite laminates, the sensitivity of the model results to the number and type of elements used to model the adhesive layer (solid or cohesive) was investigated and results compared. The limitations of the proposed approach with reference to the role of crack propagation are also discussed.