The present paper investigates the behavior and response of square and rectangular hollow sections experiencing local buckling. More precisely, the concern is here to characterize in which extent geometrical imperfections may influence the cross-sectional resistance. This is of particular relevance in the context of using advanced F.E. models in practical design, since very little recommendations or guidelines may be found in the literature. Accordingly, the present paper focuses on the influence of geometrical (local) imperfections. The effects of imperfections’ shapes, periods and amplitudes are investigated numerically, by means of suitable shell F.E. models. Adequate comparisons with measured data are provided, as well as practical recommendations for a sound F.E. modeling.