The effect of imperfections on the behaviour of a thin-walled strut designed for simultaneous local and overall buckling, the theoretical optimum design, has been illustrated by van der Neut. Local deviations from flatness remove the discontinuity between the pre- and post-buckled states, and loss of stiffness prior to local buckling may initiate premature failure in an overall mode. Imperfection sensitivity of this kind is most severe when the two modes are coincident. Thompson and Lewis have examined the optimum design of a strut with imperfections and show, in a simple example, that maximum efficiency is achieved by ensuring that overall buckling occurs before local buckling and that the actual failing load may be significantly reduced. For larger imperfections the optimum may be virtually eliminated.