Thin-walled beam structures are adopted as structural members in various fields of modem technology including aeronautical/aerospacial, naval, mechanical and civil ones. With the advent of advanced composite material systems, there is a vital need to reformulate the classical theory of thin-walled beams in a wider framework. This dissertation is intended to incorporate several essential effects which have a considerable importance for the rational design of composite thin-walled beam structures. These effects are the transverse shear deformation, the warping constraint, the secondary warping as well as the hygrothennal and the dynamic ones. The field equations of laminated composite thin-walled beams of either open or closed single and multicell cross-sections are derived through the application of Hamilton's variational principle. The Laplace Transform technique is used to obtain exact solutions. In this dissertation, the aero elastic divergence instability of aircraft wings modelled as thin-walled beams as well as the eigenfrequency problem of cantilevered composite thin-walled beams of closed cross-section are considered in the framework of a reformed theory incorporating non-classical effects. The numerical results reveal the great role played by non-classical effects as well as by the tailoring technique applied to the problems studied in this dissertation. / Ph. D.