An essential part of any flight vehicle is the structures or airframe. The structure of a conventional fixed-wing UAV consists of five principal units: fuselage, wing, horizontal tail, vertical tail, and landing gear. This chapter is devoted to review the fundamentals of UAV structural design. Moreover, primary functions of the aero-structures, primary structural members, flight loads on structures, structural materials, construction techniques, failure modes, basic structural analysis techniques, and fundamentals of structural design are reviewed.