Abstract: An extendable open frame boom structure 10 includes three longerons 11 which when the boom is extended lie in spaced parallel relationship to define three planes, A, B, C and a series of frame elements 12, arranged at intervals along the length of the boom interconnecting the longerons. Between each pair of adjacent frame elements 12, each longeron comprises a pair of foldable rod members 13, hinged at each end to a frame element. The hinges are arranged so that on movement to a stowed condition, each pair of rod members moves within one of the three planes, A, B or C. A boom system is also disclosed which includes a boom structure 10, housing 18 and a drive mechanism for moving the frame elements 12 one after another between stowed and elongate conditions.