Abstract: The present invention relates to a waterstop, 10 or 11, for restricting or preventing the flow of water across the joints of foundation elements 8, such as between or along individual diaphragm wall panels or between or along individual secant wall piles. The present invention also relates to a method of installing a waterstop at or near the joints between adjacent foundation elements. The waterstop consists of one or more longitudinal strips, 1 or 2, of hydrophilic material, wherein the or each hydrophilic strip extends vertically along the interface between adjacent foundation elements, from a position at or near the top of the foundation elements, to a position at or near the base of the elements.