An expansion device is disclosed which, when mounted on an appropriate support, can make controlled advancement relative to the support in response to activation from a remote site. The device comprises first (6) and second (8) elements coupled for relative movement along an axis defined on a support 2. An expandable chamber is defined between the elements, in which chamber is a flexible and elastic membrane (24). A resilient mechanism urges the two elements towards each other to define a minimum volume for the chamber. The chamber can be expanded to separate the elements, and hold mechanisms typically in the form of latches, are provided for selectively restricting movement on the respective elements relative to the support. Thus, activation of the expansion means advances the first element relative to the second element while the second element is held relative to the support, and subsequent deactivation of the expansion mechanism allows the resilient mechanism to advance the second element relative to the support and the first element, and to contract the chamber, while the first element is held relative to the support. Devices of the invention have particular application in the control of the growth of a bone or a bone structure, especially in the context of spinal surgery.