Abstract: A method of leak detection in a pipeline, characterized by the steps of: introducing into the pipeline (w) at an opening an initially loose fitting plug (11) having at least an expandable periphery (21); adopted for inflation by means of a flexible pipe (23); providing along the length of the flexible pipe (23) tracing means (31) for detecting the position of the plug remotely; allowing the plug (11) to be driven along the pipeline by a fluid pressure differential; inflating the plug to substantially block the flow of fluid; verifying the pressure or flow conditions upstream of the plug to detect fluid leak; and remotely detecting the position of the plug and relating it to the position of leak (x).