Abstract: A method of monitoring the alignment of pipes to be welded end to end involves supporting a reference bar extending generally parallel to, and external of, the pipes so as to span a weld position of which the ends of two pipes are to be welded, performing measurements of perpendicular distance between the bar and each pipe at two predetermined distances from the end of each pipe, determining the thickness of the walls of the pipes, determining the slope of the exterior of each pipe with respect to the reference bar, calculating from the slopes the difference between the distances from the bar to the outer surface of each of the two pipes to the weld position, so as to provide a measure of the misalignment of the outer surface of the pipes at the weld position, differencing the wall thicknesses of the pipes to provide a wall thickness difference, and subtracting the wall thickness difference from the misalignment of the external walls of the pipe to determine the misalignment of the inner surfaces of the pipes