Pipe freezing, the method and its application are reviewed together with previous research into this and closely related problems. In particular the research carried out by the Southampton University Pipe Freezing Group is summarised. The Construction of an experimental rig is described together with details of its instrumentation to measure the temperature profiles and heat flow during pipe freezing. The experimental results are analysed and a hypothesis to rationalise the observations is put forward. This is that, when pipe freezing water in vertical pipes, plug formation occurs in three phases. During Phase One the boundary layer due to natural convection exists from above the freezing zone to below it. Phase two is commenced when the growing plug has reduced the orifice sufficiently to allow the water returning up the centre of the pipe to mix with the water in natural convection boundary layer. The water in the centre moving upwards is turned near the centre section of the plug to flow downwards over the lower part of the plug. Natural convection in the upper part of the freezing zone is very significantly reduced to leave a 'sump'.