Often, light can be represented, approximately or exactly, by a complex scalar wave (psi) , smoothly varying in space and/or time. The field (psi) could be a cartesian component of the electromagnetic field or of the vector potential, or one of several scalar potentials appropriate to different circumstances. This meeting has been concerned with the line singularities of the phase of (psi) . Here, I wish to make some general remarks about these lines. Except where stated, all the remarks are independent of the particular wave equation that (psi) satisfies.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.