The invention relates to the control of leakage from ducts, such as fluid-carrying pipework, without necessarily requiring direct access to the leakage site itself; since leakage sites are often difficult to locate with the necessary degree of precision to permit access to reliably be made thereto, and moreover, even if a site can be located, it is not always convenient or economically possible to secure access thereto. The invention provides an apparatus and methods for duct leakage control wherein a sealing element (1) is introduced into the duct and is automatically drawn or otherwise guided to the locality of a leak; the element being caused, by reason of a pressure differential attributable to the leak, to move into and stem or seal the leak. The sealing element may comprise a plurality of individual members of differing buoyancy, each capable of being carried along at a predetermined level in the duct by the flow of fluid therein. The sealing element may carry a tagging device which can be used to assist in locating the leakage site.