ESDU 72003 gives the natural frequencies of vibration of built-up, flat skin-stringer structures with equi-spaced and identical stringers. The natural frequencies of such multi-span structures occur in groups, the number of frequencies in each group being equal to the number of spans. In the derivation, the frame torsional stiffness has been neglected and the frame bending stiffness has been assumed to be infinite. The skin edges at the frames are thus assumed to be simply-supported. The data can be used to predict the first two groups of natural frequencies of skin-stringer structures with four different edge conditions. The results have been presented for varying degrees of stringer torsional stiffness, for three different aspect ratios and for either one or two half-waves across the frame pitch.