Abstract: A vessel for containing or conveying a fluid comprises a wall having at least an inner layer adapted to be adjacent to or in contact with the fluid and an outer layer, each layer being comprised of a matrix of polymeric material, the polymeric materials of the inner and outer layers being differently constituted. The two layers are bonded together and the polymeric material of the inner layer has a coefficient of shrinkage during curing which is less than the co-efficient of shrinkage of the resin material in the outer layer so that the inner layer of the wall has a post-curing residual compressive stress induced therein by the outer layer. The invention extends to a method of forming a multi-layered polymeric vessel.