Abstract: A seal assembly adapted for use in sealing two mating surfaces spaced apart by a gap is provided. The seal assembly includes a sealing element adapted to be located in a groove in one of the mating surfaces. A back-up ring assembly is provided which includes a first back-up ring made from a relatively soft material and a second back-up ring made from a relatively hard material. The first back-up ring includes a body with first and second sides, with one of a groove and an interlocking projection being located on the body along the second side. The second back-up ring includes a body with first and second sides, with the other of the groove and the interlocking projection being located along the first side.