The objective of the study was to evaluate the mechanical response of composite shells containing a delamination. An analytical model was developed which consists of a structural analysis for calculating the global deformations of the structures and a fracture mechanics analysis for determining the delamination growth in the structures. In order to accurately predict the response of the delaminated shells during loading, a contact-slip condition along the delamination interface was introduced in the model through a Lagrangian multiplier technique. Based on the model, a nonlinear finite element code was developed. Numerical solutions were generated to evaluate the effect of delamination on cylindrical composite shells. From the calculations, it can be concluded that delamination significantly influences the response of composite shells. Such an effect strongly depends upon the initial length and location of the delamination, the laminate ply orientation and its curvature.