Abstract: A seal back-up assembly adapted for use in preventing the extrusion of the cap seal of an elastomer energized seal into the extrusion gap between two mating surfaces which are being sealed by the elastomer energized seal. One of the mating surfaces has a groove with side walls in which the elastomer energized seal is housed. The elastomer energized seal includes a cap seal and an elastomer energizer. The seal back-up assembly includes first and second back-up elements, which are located between the elastomer energized seal and the groove walls to sandwich the elastomer energized seal therebetween. Each of the back-up elements includes an outer member made from a relatively hard, non-resilient material which is sufficiently strong to prevent extrusion of the cap seal and an inner member made from a relatively soft plastic material which does not abrade or chafe the soft elastomer energizer.