In certain structural applications, such as stiffened panels loaded in compression, a better exploitation of the properties of composite materials requires the development of design methodologies that allow the component to work in the post-critical range. This paper describes the results of a research activity with such an objective, carried out at the Department of Aerospace Engineering of the University of Pisa in collaboration with Alenia-Transport Aircraft Group. A group of tests has been carried out on 10 flat stiffened panels, made in composite material, and the results have been analysed with two computer codes, providing useful information concerning their capabilities.