Abstract : Energy expressions and related differential equations for non-circular cylindrical shells, analogous to the corresponding relations presented by Donnell for thin-walled circular cylindrical shells, are summarized. Appropriate energy expressions are then applied to the classical buckling and to the nonlinear postbuckling problems of an axially compressed oval cylinder whose cross section is characterized by a simplified form of an expression proposed by Marguerre. In the case of classical buckling, the results show, for a range of major-to-minor axis ratio of the cross section lying between 1 (the circular cylinder) and 2.06, that the out-of-roundness has a marked effect on the critical load, and that introduction of the maximum radius of curvature into the formula for the classical buckling stress of a circular cylinder leads to good results for thin-walled shells of moderate eccentricity of the cross section. The postbuckling behavior is investigated through the application of the principle of stationary potential energy together with an approximate deflection function. The latter represents a modification of the expression applied earlier by the authors. The new results show that, in addition to the previously observed relative minimum postbuckling load, a relative maximum postbuckling load can exist. Furthermore, for controlled end-displacement loading, the large-deflection load vs end-shortening curve can correspond to stable equilibrium configurations throughout the entire loading range, whereas for dead-weight loading the region of the curve between the maximum and the minimum loads represents unstable configurations. (Author)