Abstract: A method and apparatus for the non-invasive sensing of the pressure within a pipe (or other vessel) is disclosed. An optical source produces a first light beam. This first light beam is split between a first (reference) and a second (measurement) optical fiber. The second optical fiber is associated with the pipe such that circumferential displacements in the pipe, due to changes in internal pressure, result in corresponding displacements in the length of the second optical fiber. Length changes in the optical fibers result in variations in the phase of the light emerging therefrom. The phase difference between the light beams emitted from the first and second optical fibers is then determined and related to changes in the internal pressure of the pipe.