An extendable structure which may be used in space-based applications, for example forming the body of a telescope. The structure is movable between a stowed configuration and an extended configuration, and comprises a plurality of walls arranged to give a polygonal cross-section in the extended configuration. Each wall comprises a plurality of repeating units which are connected by a plurality of hinges. Each repeating unit itself comprises a plurality of sections connected by pluralities of first and second hinges, and in the extended configuration, each of the first hinges lies between two second hinges with the second hinges having axes which are inclined with respect to the first hinge axis.