The project POSCOS within the Swedish National Aeronautical Research program (NFFP) aims at method development regarding strength prediction and sizing-methods for buckling-critical composite structures. The structural weight is an important performance parameter which is paid much attention during aircraft design. The use of reliable methods for sizing of wing- and fuselage-parts loaded in compression without introducing excessive weight will thus contribute to improved aircraft performance. The structures studied here are compressively loaded integrally stiffened flat or curved composite panels An available analytical method is discussed and a FE-based method for sizing is presented, together with design charts based on geometrically non-linear finite element analysis.