The results of an experimental and analytical study of the effects of initial imperfections on the buckling response of thin unstiffened graphite-epoxy cylindrical shells with and without a cutout, and with three different shell-wall laminates, are presented. Results that identify the individual and combined effects of traditional initial geometric shell-wall imperfections, and nontraditional shell-wall thickness variations, shell-end geometric imperfections, and variations in loads applied to the ends of the shells on the shell buckling and nonlinear responses, are included. The shells have been analyzed with a robust nonlinear finite-element analysis code for shells that accurately accounts for these effects on the buckling and nonlinear responses of the shells. The analysis results generally correlate well with the experimental results. The nonlinear analysis results are also compared with the results from a traditional linear bifurcation buckling analysis that is commonly used for shell design. The results suggest that the nonlinear analysis procedure can be used for determining accurate, high-fidelity, design knockdown factors for shell buckling and collapse. A discussion of how this high-fidelity nonlinear analysis procedure can be used to form the basis for a shell analysis and design approach that addresses some of the critical shell-buckling design criteria and design considerations for composite shell structures is included.