This paper presents an analytical and experimental study conducted for thin-walled stiffeners. The stiffener configuration is an I-beam composite section made of woven-cloth graphite/epoxy material and subjected to remote compression. A quantitative model is developed to determine the postbuckling and crippling strength. This model includes buckling and postbuckling analyses, and captures the crippling phenomenon by isolating the parameters governing the behavior. The analysis is based on von Kármán–type plate equations, the direct solution of the compatibility equation, and Galerkin's method. The method leads to a criterion for a mode of failure in thin-walled composite stiffeners. This mode is local material failure that initiates at the corners or junctures between contiguous elements and extends across the webs. The crippling criterion is based on a simple elastic postbuckling solution for biaxially loaded, thin composite plates and the use of the maximum strain failure criterion. Experimental crippling results show good agreement with analytical predictions.